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stumbling on acceleration

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JAV

JAV

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Just finished installing a fresh rebuild. All OK, except for an annoying stumbling at low RPM acceleration. Runs fine otherwise, good power, engine accelerates at rest fine, only under load at low RPM does it stumble. Stock BJ8 everything except, 60 over bore, lightweight pistons, lightened flywheel, pertronix. Timing, valves, needles, all standard. Any thoughts?
ready to install.jpgall together.jpg
 
JAV said:
Just finished installing a fresh rebuild. All OK, except for an annoying stumbling at low RPM acceleration. Runs fine otherwise, good power, engine accelerates at rest fine, only under load at low RPM does it stumble. Stock BJ8 everything except, 60 over bore, lightweight pistons, lightened flywheel, pertronix. Timing, valves, needles, all standard. Any thoughts?
View attachment 33337View attachment 33338
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Check carb oil level. Be sure your jets are centered.
 
they took off 5 lbs. Dashpot oil is 20-50. Before the rebuild this was not a problem, although since the cam had developed a flat lobe, maybe I didn't notice. Currently I plan on checking the float fuel level by the jets. I thought it is supposed to be about 1/8" down from the bridge. Is that right?
 
20w50 sounds too thick. I use Marvel Mystery Oil and it feels like a single weight 5 or 10 to me. Wouldn't hurt to try it - cheap and quick.

Second guess is mixture. Did you go through the normal carb tune-up? Balance, lifting the pistons with the pin? Etc.? I find the Colortune to be a great tool for getting the mixture just right. And do check the float level.
 
AH100M said:
20w50 sounds too thick. I use Marvel Mystery Oil and it feels like a single weight 5 or 10 to me. Wouldn't hurt to try it - cheap and quick.

Second guess is mixture. Did you go through the normal carb tune-up? Balance, lifting the pistons with the pin? Etc.? I find the Colortune to be a great tool for getting the mixture just right. And do check the float level.
Click to expand...

Used 10W at first, 20w is recommended. Went through the usual steps with the carbs, very little deviation from initial setting, ran fine, starts almost instantly. The symtom feels like fuel starvation, so I'll go into the float adjustment after recovering from mowing and plowing. Maybe tomorrow.
 
I know how you feel. Mowed yesterday. Did mulching today but managed to fit in my every-other-day 5 km run.
 
Well, as usual, it was something I did or in this case didn't do, which was verify the needles were stock. I had not changed them, so assumed they were standard UH.
 
I just went through this with Dave Nock. He feels that richer needles are now necessary due to weaker, ethanol fuel. Also, he found rust in the fuel line and fuel filter. I had the exact problem you described. Runs fine now.
 
Moss catalog lists Std as "UH" and Rich as "UN".

You could also experiment with using a dial caliper to insert the needle a few thousandths further in its seat, which would richen the carb overall. I've heard of that being done.
 
The standard flywheel is very heavy at 28lbs. A more sensible weight for normal use is around 20lbs which still provides for a smooth engine. Competition flywheels are around 10-12 lbs.
 
I've had dreadful problems with my BJ7 misfiring at over 3000, I thought it was an ignition coil because it was worse hot. In the end we took it to the local tuning specialist with a rolling road and a great deal of knowledge of these engines. It was weak and a rolling road and polishing needles to get the mixture exactly right was the only solution.

I should have know this was an issue because my Bentley is a 4.25 Lt and it has needles from the 4.5L engine.
 
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