Engine surge when throttling off, electrical?

[Adi]

Hi
I have a GS750b, it has a Boyer ignition, brand new HT caps, new standard plugs, intake manifolds are good condition and I have just replaced the manifold ‘o’ rings. The carb is standard set up with airbox.

It starts and idles well and the throttle is very responsive on acceleration through the range. The problem is when I throttle off the engine revs surge like I am accelerating and then drop off after a second or two.

If it didn’t have a Boyer I would think it a timing issue. Could this be electrical or is it more likely a carb problem?

Regards
Adi

[tom203]

Are you sure that those replacement manifold o-rings are right size?

[Adi]

Pretty much certain. Would the engine surge when throttling off if there was air leak at the inlet manifolds? They are old manifolds but look sound, no obvious cracking, it could be worth replacing them.....

[hannibal]

Throttle cable?

[londonboards]

Surging like that is almost always associated with a lean condition.

Unless it is jetted incorrectly to cause the condition (which you should check), the most likely causes are an air leak somewhere. As has been said, usually on the carb manifolds. Test this be spraying WD40 on the carb manifolds when running the bike and see if the rpm changes. If it does there is a leak.

Other areas to look at area the petcock which can leak air into the vacuum tube (and thus into the carbs) and the vacuum hose itself. Also the air box needs to be properly sealed and contain the standard airfilter.

If none of that cures it, the carbs will need stripping and cleaning and the float heights measuring carefully. If they are incorrectly set, and not allowing the float bowls the fill sufficiently, that would also cause a lean condition.

Presumably the valves are adjusted correctly? They wear and get tighter (yes it sounds odd). So over time the gap gets smaller even to the point that the valves won't close and seal completely. That could cause incomplete combustion and mimic a lean running or surging engine.

[Gorminrider]

just a thought since carbs were covered very well by LB- Is there a mechanical advance mechanism on this Boyer system? are the weights possibly catching something ? perhaps a timing light might show something.

[Adi]

Hi All
Very helpful suggestions all round, thank you.

I am now aware that I should have put a bit more detail into stating what else I had done on the bike in my original email, so I will do my best now!

The throttle cables are spot on, smooth as silk.

I checked the valve clearances just after I bought it last year and replaced a couple of shims to get it all within tolerance, so I’m happy with that.

I did check the manifolds with WD40, as I had researched the very comprehensive threads on here and read this suggestion, there was no surge in the revs when I did this. However, I did also read on one thread that this may not entirely rule out leaks on the manifolds so I may replace them anyway.

The air box suggestion is interesting, it has what looks like the original air box rubbers on it, they are in good condition but could perhaps be an issue. It has a standard air filter in good nick and oiled properly.

Similarly, I checked and set the float heights previously and got them within recommended, can’t remember what it was exactly but it was something like 26mm, service manual guidance and I checked and confirmed the fuel levels against the carb bodies with the clear hose run off the float bowl drain plug.

Definitely jetted correctly, again I checked this when I had them apart to adjust the float heights.

After I balanced the carbs I put a colour tune on it and set it up so that should be ok.

I hadn’t thought about the petcock idea, I will check the vacuum pipe, I know it is rather stiffened up with not much give, if it ever was soft pliable rubber, it certainly isn’t now! I have also had it on and off many times due to messing about with the carbs.

The timing idea is something that occurred to me but as I said previously because it had an electronic set up I set that thoughtto one side. Given what you said and the fact that I have not had it apart to check the advancer the idea deserves a revisit. I know that the Dyna system keeps the original advancer, I imaging the Boyer would too.

So, with all the excellent advice, in order of cheapest first, the plan is:

1. Replace the vacuum tube on the petcock and strip the tap to check out the rubber membrain to ensure it is not split.
2. Take the Boyer timing plate off and pull out and check / oil the advancer, refit and get a strobe on it, (haven’t used one of those for years!)
3. Replace air box rubbers.
4. Replace inlet manifolds.
5. ...........
6. ...........
7. ...........
8. Replace bike!!!! 😀🤣😀

Good job guys, thank you for the advice.

I will let you know how we get on.

Regards

Adi

[Adi]

Hi
So a quick update.
I have replaced the vacuum hose and this made no difference, engine/revs still surging or running on for a couple of seconds after throttling off.

I got a strobe on the timing plate and found that at 1000rpm the F1-4 ATU timing mark sat spot on the crankcase index mark. However, when reving to over 2,500rpm the plate did not advance right up to the ATU advanced timing mark.

As I read it, the manual indicated that at 2,500rpm the ATU advanced timing mark should be reached, is that correct?

Q. Based on this can I assume that I have an ATU unit problem?

I took the ATU out and found that the springs were quite loose, but it is a little difficult to judge how loose they are supposed to be. I know they have to allow the weights to move out with the centrifugal force so can’t be too tight.

Q. I would like to just replace the springs for certainty but I have had a look online but have been unable to find anywhere to source the springs. Does anyone know where I can buy them?

Once I have replaced the springs, if the problem persists, I don’t mind replacing the air box rubbers and manifolds but don’t want to do this before I have ruled out the ATU.