Baffled - In need of great minds

[frosty5011]

Bike - 1978 GS750E
Carbs - 26 mm VM
Air Filter - stock, foam - with stock airbox
Mileage - 12500

Situation - Received bike in non-running condition. Did complete carb cleaning. Put float height at 26 mm (Earl recommends 25-26 mm). Used standard air mixture screw and fuel pilot screw settings. Installed new plugs. Changed oil/filter. Installed fuel filter. Adjusted timing (static). Bike started right up. During idle and normal running, carbs were running rich (plugs were all black), looked like they were about ready to foul.

Took off carbs again, re-set all float heights to 27mm. This time also made sure some previous person didn't originally have re-jetted for some other exhaust/filters/etc. Needle is in middle clip (standard), and the main jet sizes are #100 (I thought 102.5 was standard???). Bike starts right up, sounds like running on all 4 cylinders. Grab pipes, and find out #1 pipe is not getting hot like other 3. Can let bike run for 5 minutes, and can still hold on to middle of header pipe with bare hand (and no, I'm no masochist). Pull plugs, all seem to be OK now, except #3 plug looking just a little on dark side (brown to black), and #1 plug seems to be "oily" (not oil fouled, it's not turning black ...... in fact, it's turning no color at all). Thought at first it was gasoline, but doesn't seem to evaporate like gas. Did this with 3 different plugs, same result.

Now, here's what really confuses me. Just for hell of it, I decide to pull plug boots while it's running. #1 pipe, after several minutes still only warm, but when I pull it's boot, significant decrease in idle speed....hmmmm, seems to have combustion after all. But, when I pull #3 boot, there is no effect whatsoever.....but pipe is just as hot as #2 and #4.

Any ideas???? suggestions???? :roll:

PS - Petcock works flawlessly (will not leak a drop without vacuum), am doing this with petcock set to RESERVE, since very little fuel in tank, and float valves all close leakfree.

[Anonymous]

I think it would be a good idea to do a compression test to see if #3 is up to specs. You could also pull the plugs and turn it over to see if you get a fat blue spark from each plug. If #3 is orange and weak you may have an electrical problem. I would try switching coils and wires and even the spark plug boots with each other to troubleshoot the problem. I hope this helps.

Chris McCarron

[MichaelHoward]

Frost Wrote:

Baffled - In need of great minds

Well, You Won't find it here!! Sorry, too good to pass up.

[AOD]

pull the plug wire off #1 and put in a test spark plug. set the plug on the engine block (so its grounded) and start the engine. make sure #1 is getting spark.

if its getting spark, its not getting fuel. check your fuel pilot screw on the bottom, and maybe your air mixture.

~AOD

[frosty5011]

Well,

I had already, before posting, made sure I was getting blue spark on the affected cylinders yesterday. I don't have a compression tester, so was unable to do that. Bike is not yet registered, so didn't want to ride it down to the garage on the other side of town to have them do it.

Being the impatient type, I did a few more things. Instead of doing one thing at a time, and seeing what difference each step made, ...........like I should, I put in different plugs again. After testing resistance between boots, I saw that reading for boots/coil between #2 and #3 was a little low (27K), whereas it should be between 30k-40k. Was then taking the boots off of the ignition wire, to see what the individual boots were reading.....I found out that #3 boot was barely attached to the wire......not screwed in tight at all, and fell right off as I was prying the little rubber cover off of the plug top. So, I took all boots off (7.4K resistance on all of them), cut off 1/2 inch of each ignition wire, and screwed all boots on "firmly". Checked resistance again between the #1 and #4 boots, as well as #2 and #3 boots. No change in value - 35K for 1-4, and 27k for 2-3.

Started the bike again and took it for a ride of 5 miles, drove beautifully. All 4 pipes nice and hot.......about a 3.5 on the "blister scale". Did a plug read, and now see that #1 is running rich.....black, sooty film on it.

Next step is (still have to check the other 3 plugs, forgot to to that before posting this).....is to lean out #1 some more. Question is, should I raise float even more (already at 27mm, Clymer calls for 23-24mm, Earl calls for 25-26mm)....or should I raise the needle a notch instead?

[AOD]

sounds like #1 is a touch rich.

[moto_dan]

Just curious. What brand of plugs are you running?

[Anonymous]

I would fill it up with gas and take it for ride and see if it cleans up. Sometimes things just need to get seated after sitting for a long time or being taken apart.

[frosty5011]

Plugs I have tried are either the NGK B8ES, or the Champion equivalent. Will have to eventually get this thing out for a longer ride - just haven't done so yet, because I didn't want to register it and get tags/insurance until I had it running like I want it (plus, it needs a new rear tire).

[KEITH KRAUSE]

I think adjusting the float height beyond spec's is just compensating for another problem and raising the needle in #1 will make it even richer. To lean it out, you lower the needle by raising the clip. But to do this to just 1 carb is also compensating. Something else is allowing too much fuel or not enough air. Not enough air could be a partially clogged air-jet or passage clogged by dirt or a piece of o-ring from the side air-screw. The #1 carb may be out of sinc with the other 3? Too much fuel could be a worn float valve,(I've seen them pass a test and look 'OK' but still fail in actual operation) a bad o-ring on the needle-jet/bleed pipe,(what the main screws into) a worn needle-jet or jet-needle, or even a loose jet. When you say you used the standard pilot and air screw settings,do you mean they were not touched? Were they still coated with that sealant from the factory? If they look like they have been turned,which can be difficult since the sealant can flake away over time anyway,I would try adjusting these also. Just a 1/4 turn on the pilots can change plug color and they have an overlap effect on the jetting up to about 60 mph in 5th gear. Turn them in to lean,out to richen. Count as you turn them in and seat them LIGHTLY,then re-adjust,or just turn them in a 1/2 turn if you're not keeping notes on their position. The side air-screws are the opposite, in to richen and out to lean. They are generally about 2 turns out. They may also be sealed. If not,try turning #1 out a 1/2 turn. They are not as sensitive to adjustments as the pilots are. I would try adjusting these screws before over adjusting other things. If #1 screws are not sealed,and plug is very black/sooty,try a 1/2 turn in on the pilot. You might try turning the air-screw out a 1/2 turn at the same time. Get a plug read and adjust again if it looks like it's working. If not,you can always put them back where they were. And even though you don't want to hear it,you need to put the float level back to factory spec's.
PS:My '79 1000 has VM26 also,but the stock mains were #95. Seems odd that a 750 would use bigger mains. Are you sure they are correct?

[frosty5011]

I tend to agree with you, Keith. I don't really want to lower the needle on just one carb, I think they should be uniform in that respect.

As far as the carb cleaning - the carbs were 100% disassembled. All needles, jets, floats, o-rings were removed. Each body lay in carb cleaner for several hours, and all air passages were blown out (in the body, as well as the one in the float bowl). Choke assemblies had been taken off and cleaned by hand (not dipped). I know you cannot synch by eye, but I did see that one of the slides was 4-5mm off, compared to the others. (Remember, this was a non-running bike when I got it) So, I set all slides to where they were "equal" in height.

For testing the floats; the way I do/did this is as follows: I have the complete carb assembly (minus the float bowls) held upright with the help of a small vise on my workbench. I have a fuel hose attached to the fuel inlet and a small tank. I then put shims underneath the floats, just enough to get them to their closing point. Then I let the fuel flow and observe for any leaks around the float valves on the 4 carbs. Once or twice, I'll remove a shim, let a little fuel through...then replace it, just to make sure it seals....even when wet. This set passed with flying colors.

The jet size does have me a little confused now. Earl stated in an earlier post that 102.5 was the correct size (my Clymer doesn't state the jet size), and JoJo also had a manual that stated the standard size was 102.5. It is strange that your GS1000 has 95's. As stated earlier, my bike had #100 in it (and they looked pretty rough). JoJo had mentioned the possibility that maybe a previous owner used some old jets that he maybe "drilled out"......this is seeming like a good possibility. I have 1 new #102.5 that I will put in the #1 carb to see what a difference it makes. Maybe that'll clear why it's running so rich. By "standard" settings for the mixture/pilot screws, I meant not as previous owner had them, but 1-3/4 to 2 turns out for idle air, and 1-1/2 to 1-3/4 turns out for the fuel pilot (standard as in according to the "How To Set Carbs, by Earl" manual.

Let me try the new #102.5 jet and see if that makes a difference. If it does, (and makes it leaner) - then we'll know the #100 jets are bad, or have been drilled out. Will keep you all informed.......and thanks for all the responses.

[KEITH KRAUSE]

Don't let me add to any confusion regarding the main jet size,it's just a thought. The other guys probably know better,I have those carbs but not a 750. Those float valves can be a real pain to 100% diagnose. Twice in the past I have just replaced so I could be sure. I have found just taking them 'apart' can mess up how the 2 parts have seated. And if you mix them up while cleaning, the chances are worse. Are you sure the o-ring around the needle-jet is good? If the carbs were sitting the o-rings usually harden. You've got room for adjustment on both screws. The pilots are set at the factory to allow for differences in each cylinder,they are not simply set uniformly. On stock bikes I have seen them turned out 1/2 to 2 turns. The side air-screws are less sensitive but are also intended to fine tune each cylinder. If you're still rich,I would try adjusting them as I said earlier.

[Don Lobacz]

I thought that 97.5 was stock for the 8-valve 750 (I'll check tomorrow on the fiche). I have seen pilot jets drilled out, too. A reamed needle jet is a real possibility. I would swap all the lower half components from carb #1 with those in carb #2 and see if the condition switches carbs. Do the top end if it doesn't. If nothing changes then you may have a plugged passage or some other odd problem with the carb body.

[KEITH KRAUSE]

Just had another thought,about the main jet swap/test you're planning. You say you rode it around for 5 miles. Unless you were at about 2/3 to 3/4 throttle and more, the rich read you are getting is not the fault of the main. If you were just riding normal the reading will be from a combination of the pilot circuit and the jet-needle position. The pilot circuit has an effect up to about 60 mph in top gear,a lot more than some people realize. Try the screws. Figuring out your main jet size is a seperate thing.

[wrench]

Frost Wrote:

Baffled - In need of great minds

Well, You Won't find it here!! Sorry, too good to pass up.

Good one mike
:
John.