750 SF Poor carburation

T

Trashers

New member
Location
Totnes, Devon
I have a new to me early 1971 750SF which has not run for many a year. Sadly this meant that the fuel tank was rusted up and the carbs were full of gunge, however I have de-rusted the fuel tank and had the crud encrusted Dellorto 30mm Vhb's stripped down and in my ultrasonic cleaner for good clean. At some time in its life has had some money put into it as it has has been nicely repainted and has had a complete rewire, which included the installation of a Moto Witt DMC for ignition.

The Problem

The bike starts and ticks over nicely, however it hesitates and stumbles when pulling away. once its running it it continues to hesitate and stumble on small throttle opening up to 3500, give it a bit more throttle and it clears itself and goes nicely. A plug chop shows the right cylinder is running very lean and the left is oily/rich. Main jet is 125, pilot jet 55, needle is set on the middle clip. I have tried new plugs swapped over plug leads and coils - all with no change. I have then upped the main jets to 132 but no change when I do another plug chop. All cables to the throttle/choke have been checked to ensure there is slack in them.

I am now at the end of my "what do I do now list" so I would be very grateful if anyone with far more experience on these bikes could figure out what is amiss and a possible fix.
 
Small throttle opening so nowt to do with main jets!
Usual suggestions for PHF carb I think can be read across to VHB.
Are pilot air and fuel passageways truly clear of crud? How did you check?
Correct float level is critical.
Atomiser jet and needle wear and benefit from replacement.
Choke plunger rubbers get hard with age and don't seal well. Are the plungers fully seated with choke off?
Do the pilot mixture screws have o-ring in place?Does adjusting the screws affect engine running? - it should.
 
To add to the above;

Are both slides correctly orientated? Sounds weird, but it can/does happen with the VHBs! Idle is only slightly affected when one of them is inserted back to front, pick-up and transition is totally out the window. WOT also only slightly affected.

Stock engine should run nicely on 125 mains.

piet
 
Main jets at 132 is just chucking fuel away.
125 is the norm.
Oily or rich on the left?
You probably have several problems starting with insuficient fuel at idle, at least on the right.
Check and double check: float heights, choke rubbers, throttle and choke cables. Make sure jets are free of course.
Paul
 
Guys, thank you very much for taking the time to respond to my query. In response, Sock - Haven't yet got to balancing the carbs with a vacuum meter, i have only measured it by eye at the moment, but its close. Rob - I believe the fuel passageways are clear of crud, as I have done them three times and blow them out with a air hose. I take your point on the atomiser and needle wear, they look fine but worth replacing as well as the choke rubbers. Pilot mixture screws do not have o rings in place . Read somewhere they shouldn't have! Adjusting screws do not make an appreciable difference. Piet - Slides are correctly orientated - I downloaded the manual to make sure I was rebuilding the carbs correctly. Paul - with you on 132s chucking fuel away, just though I would try that to see if it make a difference to the lean running side when I did a plug chop. Checked everything you have noted but will go over it again.....From the responses it seems like you all think we have issues as no one has mentioned anything to do with ignition or timing. So I will go back to the start again, order some new parts as noted and see if that improves it.
 
New bike to you ... trust NOTHING!! Just start at the carbs, move to the ignition and take it from there. No need to use gauges to set carb balance on a twin IMHO. Definitely need o-rings on mixture screws to eliminate another possible issue ... spring/o-ring/washer in that order. If adjusting has no effect, something isn't right ... eliminate choke system as a potential contributor to over-rich idle. ignition: check advance is free to move, points and timing are straightforward. Tappets - simple to adjust.
 
yes to all the above and I would add to completely replace all carb internals from a rebuild kit and use a vacuum gauge for syncing and check for air leaks on the mounting collars. stock air box or pod filters and new fuel spigots? Are the slides smooth w no scoring or wear marks? check the settings on the electronic ignition which others have had issues.
 
Trashers said:
Pilot mixture screws do not have o rings in place . Read somewhere they shouldn't have!
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piranha-bro2 said:
Definitely need o-rings on mixture screws to eliminate another possible issue ... spring/o-ring/washer in that order. If adjusting has no effect, something isn't right ... eliminate choke system as a potential contributor to over-rich idle. ignition: check advance is free to move, points and timing are straightforward. Tappets - simple to adjust.
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With the VHBs there were originally no o-ring and washer on the idle screws and concern is that adding washer and o-ring may mean that the screw does not go all the way in so that screwing it all the way in then back to a starting point of around 1 and 1/2 turns out may mean you're really much more out than intended. While the screw part 7449 is the same for VHB and PHB etc perhaps the difference is in the body casting and drillings.
No effect when screwing that idle/pilot mixture adjustment all the way in and out suggests a blocked passage.
 
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It starts and ticks over nicely, does not point to blocked idle circuit.
To synchronise, get it idling evenly, you can screw one throttle stop in as you screw the other out and you will clearly hear when it is even. Then stop it and check with your finger and eye on the slides or even hear as the slides drop back onto the stops together, and they are synchronised. I never use vac gauges on these twins.
Pickup from idle is slide cutaway, (the slides do have the same number?) from there it is needle and needle jet up to three quarters. It is a good idea to put some tape or something on the throttle and grip with marks for quarter, half, three quarters (from there up is main jet) so you can see if it is carburettor related as slide position governs what is in play. If it makes no difference to the poor running regardless of the amount of throttle I would be looking elsewhere.
When you say plug chop, what do you mean? Generally a plug chop is running under load at full throttle (main jet) and then shutting off and hitting the kill switch, rolling to a stop and removing the plugs, but there is debate about such plug readings with modern petrols. If you mean checking the plugs after a riding it, that is different matter. Any checking of this kind on these type of carbs I always do with no air filters.
Does it run unevenly standing in neutral or only when you are riding it?
An oily plug would have me doing a compression check.
As you are quite certain that the carburettors are clear of crud (checked by squirting carb cleaner through passages to see if it comes out where it should), and jets etc are the same each side (you could even swap the carburettors from side to side to see if the plug check follows suit), you could run a strobe light and see that it is advancing and firing evenly as expected both sides (I am not sure if a DMC runs two coils or a twin pole coil). It could be vibration related, a poor contact somewhere along the line.
 
Be aware that 125 stamped on new or used main jets can't be trusted! Always check for correct size.
Gerald
 
People regularly drill jets; I have done it. I couldn't get a bigger leak jet, bigger reduce Acelorator pump squit volumes in FCR Kehans, so I bought the correct size number drill and used the tiny chuck in my Dremel to drill it. That was a 0.75mm number drill up from the standard 0.35mm. Drilling idle and main jets is easy as, at least use your calibrated eyeball to compare with what you have.
 
Every day is a school day with old bikes..

Just being doing some detailed checking on VHB carbs, and it turns out there are many variants. The carbs on my bike I are VHB 30CS/CD which after a bit of investigation are not standard, it would appear they are Guzzi carbs and have accelerator pumps. Standard on the 1971 SF would appear to be VHB AD/AS. I have read a few Forum reports about the difficulty in setting up 30CS/CD carbs which kinda points to the "wrong" carbs that are the issue and possibly a malfunctioning or not needed accelerator pump causing odd symptoms. Thoughts?
 
Excessive squirt volumes from Acelorator pumps can cause a big bog or flat spot. For triples, it's supposed to be from memory, 5cc from 20 activations, don't quote me though. In my experience, they have a place, you can get a bog from too much fuel being squirted or not enough when you want hard acceleration. 75% of the tuning done on modern carbs, FCRs is setting up accelerator pumps. But plenty here deactivate then totally on Delordoes, and like the result. That's easy to do. You knock the pin out of the plastic activation arm that's on the carb top. Take care not to lose the tiny spring under that arm. I have tried it with and without, and prefer it with.
 
Vince said:
Excessive squirt volumes from Acelorator pumps can cause a big bog or flat spot. For triples, it's supposed to be from memory, 5cc from 20 activations, don't quote me though. In my experience, they have a place, you can get a bog from too much fuel being squirted or not enough when you want hard acceleration. 75% of the tuning done on modern carbs, FCRs is setting up accelerator pumps. But plenty here deactivate then totally on Delordoes, and like the result. That's easy to do. You knock the pin out of the plastic activation arm that's on the carb top. Take care not to lose the tiny spring under that arm. I have tried it with and without, and prefer it with.
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Thanks Vince, I am going to deactivate the pump and see what results I get. However I having a feeling I am going to be searching for some AD/AS as I like the original look.
 
I gave up on the Guzzi VHBs after endless hours of faffing around with them, not even disabling the acc. pump brought decent results. Still don't know exactly why though, basically the same carb as the Lav VHBs.

At the time, I thought the pumps might be an advantage for my long-stroke engine, turned out the contrary was the case.

piet
 
sfcpiet said:
I gave up on the Guzzi VHBs after endless hours of faffing around with them, not even disabling the acc. pump brought decent results. Still don't know exactly why though, basically the same carb as the Lav VHBs.

At the time, I thought the pumps might be an advantage for my long-stroke engine, turned out the contrary was the case.

piet
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Thank you very much for that information Piet, that is very helpful.
 
My SFQ FCRs are just fine without the accelerator pumps, Vince - I'm sure most dirt bikes and probably many multis need them. i can induce a bog if i grab a handful at about 3k (big 39mm carbs), but it's very rare i ever need to do that. Definitely never needed them on the SF.
 
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