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My friend's 350 engine had an issue with lots of blow-by. Lots, enough to blow thru most of the gaskets and soak the engine. Since it only had 4K on it (new short block from reputable builder) I wanted to find the problem. Pulled the engine, tore it down and found nothing out of the ordinary. Pulled the pistons out and they looked as new except the rings had not seated. They looked new. Took the piston to my favorite hot rod/engine build shop to see what they thought. They suggested the engine had too much fuel and washed the cylinders preventing the rings from seating. It has a new 670 Street Avenger vac secondary. I honed the cylinders, put new iron rings in and began the reassembly. Still no clue as to why the engine was that rich. After installing the engine and setting the carb on, I set the throttle bracket on. It bolts to the two left carb studs. The bracket seemed to sit a bit high on the front stud so I pushed it down......the bracket pushed down on the accelerator pump lever until it was almost all the way open!! FOUND IT. I got a longer carb stud and a spacer, problem solved. I have not encountered this and I thought I would share. Lots of work involved for such a simple error.
Last edited by MasterDave; 05-21-2018 at 10:33 PM.
......... They suggested the engine had too much fuel and washed the cylinders preventing the rings from seating. ................
Interesting! In my mind, I would think the fuel washed cylinder walls would lead to accelerated wear of the rings and walls due to the lack of lubrication. Thanks for posting this.
Very nice for sharing this information for sure! I'm just trying to understand-----if the pump was being held down how would it keep allowing fuel to be pumped?
Or in other words, imagine you were on a long full throttle run. The accelerator pump arm would be down for the full run. But once the initial 30cc of fuel in the pump diaphragm had been delivered the accelerator pump circuit would stop pushing fuel.
Again, thanks for the post Dave. Not trying to make any waves-----just ask'n.
Very nice for sharing this information for sure! I'm just trying to understand-----if the pump was being held down how would it keep allowing fuel to be pumped?
Or in other words, imagine you were on a long full throttle run. The accelerator pump arm would be down for the full run. But once the initial 30cc of fuel in the pump diaphragm had been delivered the accelerator pump circuit would stop pushing fuel.
Again, thanks for the post Dave. Not trying to make any waves-----just ask'n.
Once the 30cc's is gone (that is used to get you going), the vacuum would keep the engine drawing fuel. It's ok at full throttle but not at closed or partial openings. That whole accelerator pump system being almost wide open would make for a super rich condition, especially at idle. There were prior attempts to tune and reduce the excess fuel to the engine, i.e. idle fuel screws, idle adjustment but there is no way to set up a carb in this configuration. I'm not a carb expert but I just assume that the pump orifices open always would just dump fuel into the intake.
I'm pretty sure the pump arm/linkage just pushes the piston-----or in the case of a Holley, the diaphragm.
I could be wrong but the arm/linkage doesn't open any orifices.
Idle or wide open there wouldn't be any more vacuum pulling at the accelerator pump circuit if the linkage was at rest or bottomed out. Or any more chance of a siphon.
And I'm totally certain of a chance I'm full of it too!
I'm pretty sure the pump arm/linkage just pushes the piston-----or in the case of a Holley, the diaphragm.
I could be wrong but the arm/linkage doesn't open any orifices.
Idle or wide open there wouldn't be any more vacuum pulling at the accelerator pump circuit if the linkage was at rest or bottomed out. Or any more chance of a siphon.
And I'm totally certain of a chance I'm full of it too!
My friend's 350 engine had an issue with lots of blow-by. Lots, enough to blow thru most of the gaskets and soak the engine. Since it only had 4K on it (new short block from reputable builder) I wanted to find the problem. Pulled the engine, tore it down and found nothing out of the ordinary. Pulled the pistons out and they looked as new except the rings had not seated. They looked new. Took the piston to my favorite hot rod/engine build shop to see what they thought. They suggested the engine had too much fuel and washed the cylinders preventing the rings from seating. It has a new 670 Street Avenger vac secondary. I honed the cylinders, put new iron rings in and began the reassembly. Still no clue as to why the engine was that rich. After installing the engine and setting the carb on, I set the throttle bracket on. It bolts to the two left carb studs. The bracket seemed to sit a bit high on the front stud so I pushed it down......the bracket pushed down on the accelerator pump lever until it was almost all the way open!! FOUND IT. I got a longer carb stud and a spacer, problem solved. I have not encountered this and I thought I would share. Lots of work involved for such a simple error.
Blocking the accelerator arm in it's nearly full travel position would have no effect on the way the engine runs other than a slight hesitation on acceleration at low/mid RPM. It would not make the engine run rich.
There is another issue causing the engine to run poorly/rich.
Make sure your air bleeds are clean,if they are dirty or blocked it will run very rich. The Excelerator pump has nothing to do with it as long as the check valve is in and working. Make sure the gaskets for the metering blocks are installed correctly and tight .
Interesting thoughts. Thanks. I'm fairly certain that the new Holly carbs have a blowout protection for the power valves. Vacuum at idle is 18-19" (or whatever it's called) so a 6.5 PV (which it has) is good to go. The engine has no blow-by or gas odor now. Other than a hone and new iron rings, the only thing I found was the accelerator pump rod in the almost full open position. Just sayin'........
Interesting thoughts. Thanks. I'm fairly certain that the new Holly carbs have a blowout protection for the power valves. Vacuum at idle is 18-19" (or whatever it's called) so a 6.5 PV (which it has) is good to go. The engine has no blow-by or gas odor now. Other than a hone and new iron rings, the only thing I found was the accelerator pump rod in the almost full open position. Just sayin'........
If the engine runs good now, you probably corrected something else along the way because holding the accelerator pump lever in full on position definitely wouldn't have made it run rich or bad.
If the engine runs good now, you probably corrected something else along the way because holding the accelerator pump lever in full on position definitely wouldn't have made it run rich or bad.
It's possible that the previous owner did not seat the rings properly causing the blow-by. Not sure if that would have any affect on fuel odor tho. Thanks for the info guys......
I had that happen to a Harley Davidson eng years ago. Drove me nuts. I gave in and pulled the jugs and pistons and brought them to my eng. builder. He took one look at the jugs and said no problem. He then sanded them with ermry cloth and told me to put it back together and it will be fine. I did what he said and damned it worked. Rode the bike for 13 years and never had a problem again.
05-21-2018, 10:32 PM
Found the issue/interesting (long)
Dennis
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