carbs. What have I done?

[Alan G]

Hi all,

VH500 with an AMAL 376 29C monobloc carb. In a 1946 chassis, with a later engine, iron head, twin port.

I recently had the head off the bike to change push rod rubber seals. So valves were adjusted, but I have not done anything to the electrics or anything else, so I am pretty certain my issue is with the carb. Before doing the work, the bike started and ran well - but was maybe lacking in power meaning that cruising above 50ish was a struggle. This may be as it should be, but I did expect it would be stronger in top gear as others have said that 60mph cruising was the norm. They may have been lying...

I stripped the carb, removed the remains of an insect exoskeleton from the needle jet area, gave everything a high air pressure blow through and was hoping that once reassembled I would have unleashed another few horsepower.

Started easily on first or second kick, but when opening the throttle it bogs. Opening it very slowly it is slightly better but once up to about 1/4 throttle the thing just dies. Starts easily enough again, but can not run anything like it should. No way is it rideable. Something basic is wrong.

So any immediate "I've done that!" thoughts?

Have I blown away a seal or spacer I didn't see? Have I got the float level an inch out (never looked at float level, just reassembled). Any thoughts?

Thanks in advance for any input.

[Roger Gwynn]

Put the insect back in !

At least the problem should be obvious, check everything that you have done, there will be something you will have done wrong.

[Simon.Gardiner]

!/4 to 3/4 is needle, so check that the needle is still in the right position in the clip.
Bearing in mind some fairly recent advice in the Forum about wear affecting the needle and needle jet, and poor quality control potentially affecting all the jets, you might be advised to replace the needle and jet as a matter of course anyway.
(60mph cruising is conservative, I used to use 70-75 on my '51 rigid. We did have proper petrol back then though... )

SG

[nevhunter]

Make sure the float and needle is working properly (freely) and doesn't bottom out and reduce flow the and fuel flows well through the fuel line. Clean the bowl out fully and get a new gasket and make sure there's no fluff or crud in the Main Jet. Nev

[ian williams]

hi Alan ;
have had similar things with my NH and 376 carb where it hesitates on first 1/4 of throttle .
go through the pilot jet parts and its orifices in carb especially the very tiny hole in the lower carb mouth just above where pilot jet is located , make sure thats all clean and not blocked .
most of the time on mine its something to do with the pilot jet orifice in the carb mouth being blocked and i have to clean it out quite often which i think is down to the modern pump fuel causing deposits in the carb.
ian

[charles.bulloch]

They are not lying about 60mph cruising btw. Maybe slightly faster, if they can hold on!

[Alan G]

I took the carb out again today, checked it all and it is clean as a whistle and put it back in again. Still the same. I was wondering if the float height is too high - will need to get a clear plastic thing to make a cover with so I can see it working. Anyone got one spare they are willing to sell or lend?
Another thought was that the valve wasn't closing and thus flooding - but there is no fuel leaking when the engine is off - so that seems unlikely.

Looking in the back of the carb when opening the throttle with the bike running i can see fuel condensing and dripping of the front of the slide - I guess this is normal, but maybe is a symptom of being too rich.

The numbers stamped on the jets are correct going by the book, only thing I didn't check/measure was the needle - but it felt smooth with no sign of wear.

I guess it is possible and I have done nothing wrong, except moving the dead insect which has resulted in more fuel flow through the needle jet - but seems unlikely. This may tie in with my lack of top end performance if it was running lean on full throttle.

So next step is float height. If that is fine, I guess I am into swapping jets and needles.

[Alan G]

Quick update, ran the bike with the fuel tap off to see if the drop in fuel level fixed the issue. It did not. So I am thinking it is not float level related.

[nevhunter]

Float level mainly affects starting and idle . Total flow rate Flow into the carb and the Main jet size affect the top end. Check your fuel cap Vent. Nev

[ian williams]

hi Alan;
your float level remark prompted my mind .
something i forgot to mention , STAYUP plastic floats , they swell up slightly and when trying to come up they stick inside the float bowl on the left side screw boss .

amal carbs [burlen] now machine off some of the screw boss material to give the float more clearance there so they dont stick .

it is most likely that the new fuel we buy is the cause of the stayup floats swelling up slightly and sticking .

i have had this issue appear last year on my 376 carb but i changed the float to an old brass float and that has more clearance than the plastic float does .

my carb was made in 2014 and does not have the machined screw boss

i dont know how the original amal plastic floats fare but the "stayup" floats seem to suffer a problem now with them swelling up slighlty along the join line at opposite end of float to the hinge point .

my other problem with engine running badly , stuttering on first 1/3 of throttle was down to the needle jet being too big , i dropped to a 105 and it then ran fine .

hope you sort it out , i know what a pain it can be when you have a problem with carb .
ian