Hey Earlfor et. al. Exhaust & jetting ?

[Rico]

All you jetting and exhaust Gurus PLEASE chime in.

Supertrap 4 into 1 exhaust on my 78' 1000e, good or bad?
What jetting am I looking at?
Preformance boosts?

I found the header cheap and I want to know if it's worth the trouble.

If so, should I pod up the carbs @ the same time?

It's a strong ride now with 38,000 miles. (but stronger is better \/ ) What kind of improvement might I be looking at?

Like always, thanks
Rico

 

[earlfor]

Re: Hey Earlfor et. al. Exhaust & jetting ?

A supertrapp is a good system and although I have never had one, my understanding is that with the baffle system, it is tunable, so I would probably keep it.

Jetting questions on 750's and somewhat on 1150's I have experience with. I've worked on a few 1000's but never owned one, so except for a couple with pods already installed and mostly stock configuration, I have very little hands on experience with that model. Stock mains for that bike are #90's. Just ballpark, adding just a pipe, I would probably increase mains to 95's and leave everything else alone. (my guess)

Keith on the other hand, has owned one for (I think) about 15 years, so he
is the guy to talk too with jetting and modding questions for it.

Earl

All you jetting and exhaust Gurus PLEASE chime in.

Supertrap 4 into 1 exhaust on my 78' 1000e, good or bad?
What jetting am I looking at?
Preformance boosts?

I found the header cheap and I want to know if it's worth the trouble.

If so, should I pod up the carbs @ the same time?

It's a strong ride now with 38,000 miles. (but stronger is better \/ ) What kind of improvement might I be looking at?

Like always, thanks
Rico

 

[Mr. Jiggles]

You'll also need an adjustable jet needle to go with the larger mains. This will allow you to adjust the midrange mixture. When you consider the time spent it's probably a good idea to spring for a dynojet kit. I agree with Earl - the supertrapp is tunable (sp?) and worth keeping. Personally I like the sound of a 4-1 exhaust as much as the added HP. One thing to consider about a 4-1 system is whether you can keep the center stand and ifit provides clearance for access to the oil filter and drain plug.

Stay away from pods at least for now. Although they make getting the carbs on and off much easier (and give more HP) they will make the engine much harder to tune.


et al.

 

[KEITH KRAUSE]

From what I've heard, Supertrapp makes a good pipe. But in my opinion, a pipe alone isn't going to increase the performance much. It depends on your situation. If your stock exhaust is in poor shape, then you need to put something on anyway and any re-jetting is just part of the swap.
If you're itching for some more power, then the pipe has to be complimented with pods...K&N pod's as far as I'm concerned. Properly jetted, you WILL notice a significant performance increase. How much I can't exactly say, but a good 8-10 HP is possible. How usable this extra power is depends on the condition of your motor, cam timing and the way you ride.
If you just want to put the pipe on and it really does flow better than the stocker, then a full size up on the main jet will probably be good. Stock main jet is #95, so you'd need a #100. You would still need to test. Your adjustable jet needle may need to be raised a 1/2 position by using a jetting spacer and moving the e-clip. Again, you'd have to test. Your pilot circuit would just need a small adjustment to the pilot screws. You would have to take plug reads before doing anything.
If you want to pick up that extra HP the pipe/K&N's will give you, then you'll for sure have to re-jet all three circuits. To re-jet all three, you'll have to be able to work on the carbs and have patience with the re-jet. You'll also need to have a vacuum synch tool. Anytime the jet needles are disturbed on the VM carbs, the carbs must be re-synched. You'll probably want a jet kit too instead of buying and trying several mains and pilot jets. I personally think the jet kits work better because the stock adjustable jet needle is too hard to get good results with.
So that's what's involved.
With just the pipe added, you may get away with just buying larger mains and some pilot screw adjustments, but don't expect much more power.
With the pipe/pods, you may need to spend about $120 on the jet kit, about the same for the K&N's, and about $40 for a simple carb synch tool, though you should have a carb synch tool anyway. You may also find that you need to buy new o-rings, etc, for the carbs. Then you need a decent manual and patience. You WILL notice more power, but you won't go out and blow away most modern bikes.
If you need help with the jetting, let us know. There's quite a bit to know to do this job right. Hope this helps.

 

[nabrams]

Hi All,

I just picked up a 1981 Honda CB750 that had been sitting for three years. Of course it would not start until I cleaned the carbs thoroughly. Its got a non-stock 4-into-1 exhaust system with the stock carbs and airbox. It runs (I've verified that all four cylinders are firing and there are no air box leaks and the air filter is fine), but lacks power. There is not a perfect seal between the header and the pipes, so that might contribute to the lack of power. Once I get that resolved, do you think just raising the needle a millimeter on each carb (even though they're CV carbs) might get the bike to run better? Or am I going to have to get larger jets no matter what?

Thanks

 

[Anonymous]

pipe

pipe

T think the first thing you should do after mounting the pipe is decide on how many baffle plates your going to use, cause it will make a difference in the jetting and power you will produce, more plates equal better breathing, fewer plates equal more back pressure, if you got the money do the whole thing up if not put a K&N in the stock box and add a few intake holes in the filter box, but you will have to jet up for this too,

 

[Anonymous]

81 honda

81 honda

With out knowing what is in the carbs (Jets) to compare to stock if they have been changed makes it a tough call, also the pipe brand and type of 4 into 1 will make a difference i.e. megaphone vs. canister, you need to post more info

 

[Jeff Tate]

This may be of some help to alot of people. I bought a new V&H megaphone exhaust for my 1100 last summer and it came with jetting suggestions if NOT useing a jet kit for most GS models from 450's up to the 1150. Even though this comes from V&H it should get anybody in the ball park that just wants to add a 4-1 header. The weight savings alone is something to consider other than HP. Anyway for a pre 80 GS1000 they recommend removing the airbox lid and increase the main jet size to a 117.5 or 120. Also adjust fuel screws out 1 to 2 turns from fully seated.

 

[houndawg]

i installed a v&h header on my 79 my baseline was ....pilot jet17.5 ... raised needle 1 notch to #3and 120 on main jets--which i later changed to 125's [with the pod type filters ] hope this helps get you in the ballpark

 

[KEITH KRAUSE]

Hi All,

I just picked up a 1981 Honda CB750 that had been sitting for three years. Of course it would not start until I cleaned the carbs thoroughly. Its got a non-stock 4-into-1 exhaust system with the stock carbs and airbox. It runs (I've verified that all four cylinders are firing and there are no air box leaks and the air filter is fine), but lacks power. There is not a perfect seal between the header and the pipes, so that might contribute to the lack of power. Once I get that resolved, do you think just raising the needle a millimeter on each carb (even though they're CV carbs) might get the bike to run better? Or am I going to have to get larger jets no matter what?

Thanks

After you take care of the header leak, I'd check the ignition timing and then synch the carbs. Then I'd go out and test a few miles at 1/3 throttle and get plug reads. That would tell you what you need to do to the jet needle.

 

[nabrams]

Sealed exhaust leaks, still weak high-end power

Sealed exhaust leaks, still weak high-end power

Hi All,

OK, I've got the exhaust leaks sealed perfectly and the bike runs considerably better, but I still lack high-RPM power.

Here's the current set-up:
1981 Honda CB750
Black MAC 4-into-1 aftermarket exhaust (I think it's the cansiter type)
Stock air box and air filter
102 main jets
Plugs ar a light tan color.

So, I'll be recieving a main jet kit shortly with the following size options:
105, 110, 115 and 120

Should I first try the 105, or should I just jump straight to the 110 (or higher).
I'd rather the bike run slightly rich than be too lean, as long as it will not foul my plugs. I would even go so far to say that it's perfectly OK with me to jump straight to 120 if you gurus are pretty sure that won't result in fouled plugs.

Thanks in advance for any advice.

P.S. - I still love my GS450 because it's not as bulky as the 750 and it's considerably quieter.

 

[KEITH KRAUSE]

I think you should have waited before ordering a jet kit. Also, what jet kit did you buy that's designed for just a pipe mod and stock filter? I don't know of any. Using a jet kit designed for other mod's in mind is asking for trouble.
If you're concerned about the mid-range, as in your first post, you need to get plug reads at 1/3 throttle. This will tell you what needs to be done with the jet needle specifically. You did say that your plugs are running a light tan color, which is good. I'm assuming this read is from general riding/a combination of throttle positions.
If the Mac pipe DOES flow better than the stocker, then you will have to go up a step or maybe two on the main jet. Each step in Mikuni jets is 2.5
I really doubt the jet needles will have to be raised. You may have to adjust the mixture screws out maybe a 1/2 turn? It depends on testing and the plug reads.
Your problem with a lack of top end power could be many things. It could be compression, valve clearances, cams worn, etc. I assume your carbs are in good condition and have stock jetting? The filter is clean and the filter has been oiled correctly(?), unless paper. No intake leak symptoms such as higher than normal idle once warmed up?
If you're SURE the above is in good order, I would first make sure the carbs are completely clean inside. If you have o-rings in the carb manifolds, I suggest replacing them. Then I would check the float levels. Make sure both ends of each float are even first, then adjust if necessary. Then make sure the ignition timing is set correctly and is advancing. Make sure you carbs throttle plates/slides are opening completely by following your manual. Then set your mixture screws for the highest idle and do a vacuum synch to the carbs. Any of these things could cause a lack of top end power and are considered general maintenance. These things MUST be checked first. To simply assume any one of these things is good and not actually checked/adjusted, is just going to give you more troubleshooting problems. You can't accurately check the jetting if any of these things are not right.

 

[nabrams]

Kieth,

Thanks for the detailed reply. I've verified everything you mentioned except the carb synch - I have to get the tool for this and learn how to do it - this is my first 4-cylinder bike. I'm pretty sure the bike does not have an adjustable advance mechanism from what I've seen in the manual (my 1980 GS450 certainly does not), but I'll verify that too.
The bike has only 14000 miles on it (and looks it) and runs fairly smoothly, so I really doubt there's anything like valves or cams out-of-whack (though it is possible, I suppose).
Also, just to clarify: When I accelerate slowly, the bike rides solid up till about 7000 RPM. When I gun it, it hesitates and coughs but eventually will make it up to 7000 RPM. I can't get past 7000 RPM.
Also, it idles fairly smoothly and has decent acceleration till about 4500 RPM. Would this be possible if the carb synching issue was the primary problem?
Also, the jet kit I got ($29 on eBay) is only main jets (not needle or secondary). I believe I've heard people say that it is quite common to require a larger (than stock) main jet if the only mod you make is an aftermarket exhaust. Do you agree?
Anyway, I hope to have the time to try the carb synch (if you still think it's necessary) and the larger main jets over the next week or so, if the weather cooperates.

Thanks again.

 

[nabrams]

Never mind my question about "I believe I've heard people say that it is quite common to require a larger (than stock) main jet if the only mod you make is an aftermarket exhaust. Do you agree?". You answered that already.
But I would appreciate any input about my other question in bold.

Thanks

 

[Jethro]

Also, it idles fairly smoothly and has decent acceleration till about 4500 RPM. Would this be possible if the carb synching issue was the primary problem?

I just got into carb synching this past year myself. I've been surprised at what I have learned. When I got my 83 this fall, I thought it sounded perfect. It idled great and seemed right on. But I hooked up the synch gauges and started adjusting and was like, "Wow, I had no idea the synch was this far off!" What sounded like a perfect idle got so noticably better as I equalized the synch. I imagine that the carb synch (or lack thereof) can make a bigger difference at higher RPM's as more fuel is drawn in.

Carefull, carb synching can become addictive! I have done it three times since I got the bike in Sept.!

 

[KEITH KRAUSE]

Yes, the carbs synch could cause your problem. I also wonder if your throttle plates are opening completely. Normally, you follow your manual to make any fully open and fully closed position adjustments first. This would "manually" synch the carbs. Then you adjust the mixture screws and do the far more precise vacuum synch.
Your problem could be fuel starvation or poor venting too. Some easy checks are to be sure the fuel line is 5/16", the vacuum line is new, the petcock is completely clean, and the gas cap venting is clean.
Again, these are all normal maintenance things that need to be done anyway.
Do these checks, synch the carbs, and the bike should run well. If not, then things have to be re-checked more carefully.
If the Mac pipe is flowing better than the stocker, I would try one full size (5) up on the mains and test. You can make a small adjustment to your mixture screws to help with any "around town/lower speeds" issues. I really doubt any needle adjustment will be needed.

 

[KEITH KRAUSE]

By the way, are you sure the stock main jets are supposed to be 102?
What brand of carbs are on the bike? Keihins? Mikuni jets increase in 2.5 increments. Mikuni doesn't make 102, but they do make 102.5
DJ and Mikuni size their jets differently, I'm not sure about Kiehins, so I wonder what brand of carbs you have and if the E-bay jets are sized the same way as the jets that your bike has now.

 

[nabrams]

Kieth,

Yes, they are Keihin carbs, and I verified that the stock jet is 102 (according to my Clymer manual). It remains to be seen if the eBay jets are in fact for my carbs (the seller indicated that they are). I disconnected the fuel line from the carbs to verify unrestricted fuel flow (and it is 5/16").
My throttle plates move freely and open fully at full throttle (I verfied this when I had the carbs off the bike). I also verified that I do NOT have an adjustable advance mechanism (it's CDI).
I'll do the carb synch, but I suspect I'm going to put 110 jets in (when I get them) because I found someone on another site who said that was what he had on his 1978 CB750 that had a 4-into-1 exhaust. It is true that his bike is 3 years older than mine and his 4-into-1 exhaust was not the same as brand as mine, but I'm starting to suspect that increasing the jet size is allmost "standard operating procedure" for 4-into-1 exhausts. Even my Clymer manual states (just found this last night) that changing to a non-stock exhaust will likely require re-jetting.
I'm guessing the previous owner got the 4-into-1 exhaust because it was cool, but never had the patience to properly seal the pipes to the header and deal with the re-jetting. That's probably why he let the bike sit for three years. I can't confirm this guess because I bought the bike from the PO's father (he didn't have much history info).

Wish me luck. I'll let you know how it goes.

Thanks again.

 

[KEITH KRAUSE]

OK. Good luck. Like I said before, if the pipe flows better than the stocker, I would try (5) up on the mains, which would be 107. Of course, you don't have 107's, but 110's are fairly close.
And are you sure the advancer is working on the bike? Can you shoot it with a timing light and see movement?
Let us know how it goes.

 

[nabrams]

Hi All,

Well, I installed the larger jets (110) and the bike runs great now. I get great acceleration at all RPM ranges and the bike idles smoothly after it's warmed up. The plugs are light tan. All this is without the carb synch. I guess it will run even better after I synch the carbs. Luckily, I got this done before the snow storm here.
I got this bike with the intent of riding it for a bit and then selling it (I already have a 1975 Honda XL250 dirt bike and my trusty 1980 GS450), but it handles so well and now I want to keep it. I've been restoring a 1973 Datsun 240Z that will take up most of my tinkering time.
Decision, decisions - what to do?

Thanks again for the advice. Now if winter would just pass quickly so we Northeners could do some nice riding!