Popping again, looking for a little advice...

[Beaver]

Ok. I realise that I haven't done all the different tests and what not that can be done to get to the bottom of the problem, but I am a little pressed for time and I REALLY need my bike running ok by monday I am also pretty certain that with the knowledge that you gents have that you can give me a push in the right direction based on what I have found out so far.

http://www.thegsresources.com/_forum/showthread.php?t=151662

I posted this thread in autumn last year and and have not really had the opportunity to ride the bike seriously until the last few days.

We have a ´77 750, running a 4to1 marshall, pods, 125 mainjets (6 up from standard) the needles have been raised 2 notches, carbs synched, and I have adjusted the fuel mix at idle using a morgan colourtune. When I used the colourtune I noted that I got just blue at idle, yellowy orange in the midrange and blue in the top end. I have not done plug chops as I have just not had the time.

The bike is still popping in the midrange.

Another thing worth noting is that in the autumn I took it on a short 10-15 km trip to buy some foam. After getting on the motorway (still popping) the bike died just short of my destination, had been doing about 120km for less than 5 min. I managed after a bit of waiting and pushing to get it started and home (stalled once or twice on the motorway on the way home).

I just tried to do the same trip today, again to buy the foam I never got the first time. The bike stalled about 500m short of where it stalled the first time. This time I was a little smarter and checked a few things. The 2 right hand cylinders had lost spark (the exhausts were notably cooler). I could also start the bike straight away on full choke, but not get it to run with anything less than full choke. After just a few minutes I had it running normally and all 4 pipes were hot. I made it to the foam shop and kept my speed down to about 80-90 (no popping) and made it all the way home without any further problems.

I have taken the carbs home and am planning on spending my evening adjusting them as it is my guess that that is where the problem lies. My thoughts were to take the needle up to 3 notches as I can't really think of anything else to do.

Maybe also worth noting is that I attempted to change the camchain as it is a bit noisy but manged to kill my chain splitter splitting the new chain so given the fact that I need my bike running on monday, with or without a camchain rattle I decided to take that no further. But on reassembling the top of my head and starting the bike I noticed that it had developed a tapping sound that I figured could only be a valve. I then found out how to check clearances and discovered where the tapping came from, one of the intake valves had .14 gap instead of the .3-.8 allowed. Not having the shim removal tool I just took the cams out again, checked the existing shim thickness out, put the cam back in and ordered the new shim (and special tool) Upon assembling it all again the valve noise was gone and I haven't re-checked the gaps yet, but thought that this was worth mentioning in the process of trouble shooting my problem.

Any ideas? I realize that plug chops are a good idea and if necessary could do them on Monday but figured that I must have a bit to go on already...

Any ideas?

Cheers

Angus

p.s. just pulled one of the carbs apart and found I can't move the clip any further down. Do I have to go up a size in main jet then?

[Beaver]

I now have something a little more concrete to go on.

I have done plug chops at about 1/4-1/2 (it was held constant) throttle

#1


#2


#3


#4


I have had the fuel screws from 3/4 of a turn as Steve once suggested to 1, 1 1/4 and 1 3/4 turns out. I have adjusted the air screws from 3/4 turn to 3 1/4 turn and none of this makes any difference. No change in the way the bikes runs (badly), or the the plug colour.

In the city it is fine as it seems the constant change in throttle etc helps. Once on the motorway at say 55mph it will pop a little at the start and then get gradually worsen to the point where it will lose power and die. It will start again only under choke. I can also get it to limp home by feathering the choke everytime it sounds like dying. I have also tried running it at full throttle at which point the popping stops but by the time I hit about 70mph it just dies.

Any ideas? As I can see it I may have the wrong size main jet(6 sizes up, 125), or I have missed something in the rebuild process.

Cheers

[rustybronco]

Angus,

I'll give my opinion based on what I have done with similar models. main jet size is selected at 3/4~full throttle and only comes into play in those throttle ranges. I can't tell you if your main jet is spot on, but it should be very close.

what colour is the area where the insulator meets the outer shell? if it's black, your main jetting is to large and you may want to go down to 122.5's. somewhere around 122.5~125.0 should be in the ball park in my opinion.

the problem you are having is at mid throttle and is under the control of the needle and needle jet.

keeping that in mind, in regards to your needle clip position, my opinion is your clip should be placed one position down not two. for example, if the clip was on the third (middle) drop it to the fourth position. this assumes your needle has five notches as I don't know what number grooves your needles are equipped with.

one more thing to keep in mind, excessive fuel will wash the color off of the plugs. excessive fuel can come from, among other things, too high of a fuel level, incorrect AFR, or a leaking petcock (see #3 plug colour) or an improperly sealed fuel passage. you may have a combination of different things causing the variance in plugs colours you have.

lastly, have you taken any compression readings as of yet?

[Beaver]

Thanks

What is AFR?

I have 5 positions. At Cafekid's suggestion I moved it from the middle to the lowest (2 clip positions)

I did a compression test a few months ago and got around 90 across the board as I can best remember. It probably wouldn't hurt to check again though.

Another thing that might be worth noting is that I have been told to adjust the air/fuel screws until I hear the engine rpms peak but I noted no change in rpms when I adjusted them.

[rustybronco]

Air to fuel ratio. also known as A/F ratio.

put the clips in the fourth position not the fifth and check the fuel petcock for leakage.

also, you could try 7/8~1 turn on the fuel screws, 1-3/4 on the air screws as the starting baseline and tweak according to throttle response. first make sure to record where they are set at now. the best thing you can do to carbs that don't respond to the highest idle method, short of figuring out why you can't adjust them properly, is use your senses. I.E. does it smell too rich and does it respond to throttle...

[rustybronco]

are you pulling the carbs to adjust the clip positions? if so make sure to bench sync them by the eyeball* method. in my opinion it beats measuring the gaps with a drill or paper clip.

*eyeball method=set the light purchase(?) the same on all carbs with minimal slide to bore gap.

[rustybronco]

A few more things if I may.

you need to check the resistance of the coil secondaries, both the wire and plug cap values. then check the point condition, gap and timing for both sets of points. also, does one set of points show excessive sparking when the bike is running? that would be an indication of a bad condenser. any of these systems, if bad, could cause the bike to run poorly.

it appears at the outset you have more than one problem, the loss of spark issue combined with a possible bad petcock diaphragm/restricted screen which could be the cause of the bike dying at higher speeds and the fuel washed look of #3 plug.

all the systems must be working properly or you'll just be beating your head against a brick wall hoping that it will fix it's self.

And I

No change in the way the bikes runs (badly)...

I suspect you have more than one issue mate... let's try to fix it before Monday.

[Beaver]

Woah. Lots to cover there. Now that you mention it the petcock drips quite slowly when the tank is disconnected. I just thought that was normal. Also maybe worth adding is that it has 2.2 ohm dyna coils and new leads.

The carbs have been bench synced and also done with a carb sync tool after. How do I check for excessive sparking?

Cheers

Angus

[rustybronco]

You still are using point and condensers correct?

what you don't want to see is this...


see the nice bright sparking on the left point contacts? that's an indication of a bad condenser.

[Beaver]

Ok, to sum up:
Clip down just one clip
7/8 - 1 turn on the fuel screw
Check point condition and gaps
Check resistance in coils and secondaries (how?)
Wire plug cap values (again how?)
What about float levels, do they play a role i this you think?
Also the leaking petcock, is it a new petcock, or just the diaphragm (service kit)?

Cheers

Angus

[rustybronco]

Clip down just one clip

Yes.

7/8 - 1 turn on the fuel screw (bottom)

Also 1-3/4 turns on the air screws (side) as a STARTING point.

Check point condition and gaps

Also check for excessive pitting and arcing.

Check resistance in coils and secondaries (how?)

Plug cap to plug cap resistance from 1 to 4 and 2 to 3 wires.

Wire plug cap values (again how?)

By removing the plug cap from the plug wire.

What about float levels, do they play a role in this you think?

They play a large role and it might be worth the time to check them. remember your not dealing with all new parts inside the carbs.
are they the issue with your problem? dunno yet.

Also the leaking petcock, is it a new petcock, or just the diaphragm (service kit)?

Most likely a new petcock will be required, unless you can ensure the sealing surfaces are flawless. the whole point is to renew the petcock, not just replace a few parts.

[Beaver]

And a few more questions:

What are wire plug cap values? What exactly are you measuring?

I measured resistance cap-cap and between 2&3 @ 20k Ohms on the multimeter got 14.36

Between 1&4 only got a reading @ 200 Ohms and it fluctuated beteeen 124 and 150 and sometimes wasn't there at all

One of the points was a little pitted due to one of the contacts not hitting squarely. I have ordered a Dyna S electronic ignition (while I am at it) and a rebuild kit for the petcock. as I couldn't find a new one for my year of bike.

[rustybronco]

Between 1&4 only got a reading @ 200 Ohms and it fluctuated beteeen 124 and 150 and sometimes wasn't there at all

Something is wrong with the values you are getting. Retake that measurement with and without the plug caps. also try a higher range like in the megohm range.

[Beaver]

Will do. I put the clips up one on the needles, and bench synced the carbs. I haven't had a chance to sync them properly but I must say that my first reaction is that the popping in the low range 0-1/4 throttle has gotten worse. The most depressing thing with all this is that the damn thing pops the second I start it so I know that something is still not right.

I also checked the float levels and they are fine. The plug-plug resistance will have to wait till tomorrow. Trying to be patient...

[rustybronco]

my gut tells me your resistance values are off the chart..