Looking for advice on fuel problem

[ghostriders]

Looking to see if anyone can confirm or suggest something else. The problem is when I am going down the road, picking up speed to just about 45, the motor starts to cut out. Jerks the 40 pretty violently. The motor does not die, I can rev the engine just fine. Put it back in gear and does the same. I have to slow down and run it in second to third slowly. I have been driving my 40 since I completed it in March/April. During build up I replaced the fuel pump, fuel filter, and all fuel lines. I also rebuilt the carb. When I had the tank out there was rust in the bottom. I cleaned it out and put it back in. Today I filled a 5 gallon can of gas and bypassed the tank. I did not seem to have any issue's. What I think is going on is I have junk still in the tank and is clogging the pickup. What I dont get is why only at those speeds. Anybody have any opinions or suggestions? The motor is the F.5 with the Aisan carb. Thanks in advance, Dan.

 

[raydouble]

Sounds like it might be a spark/ignition issue.

The reason I say that is cause you said it revs fine, but sounds like it is cutting out under load.

Is it vacuum advance or mechanical?

 

[vk7ybi]

I had same symptoms. The filter was clogged.. If the pickup is clogged it will be the same as clogged filter.. At higher RPM/load/speed the engine uses more fuel, but at lower speeds enough can get through.. A quick stab of the throttle will not show problems because fuel is taken from the float chamber..
Drive the vehicle till it starts playing up, as soon as it does, depress the clutch and turn the key off.. Get out and check the level in the float chamber. If you have see through filters, check for fuel in there.. Might be helpful to temporarily install see thought bits at various points in the fuel line to see how far fuel is getting..

 

[Pighead]

fuel filter.

 

[ghostriders]

I have a Mallory electronic ignition but really dont think it is an ignition problem. Will go out and check the fuel filter. Maybe it will be the easiest thing this time!!! Thanks pighead.

 

[Rocky_LC]

I had the exact problem in my '74 as you. My issue was related to a "relined" tank, which was deteriorating.

All of the crap that was coming loose was clogging filters like crazy.

As I understand it, there may also be a little "basket" filter in the carb as well. This could be clogged, or the fuel filter.

Anyway - I broke down and got a new tank. Cleared up all my problems. I would have rather just changed out the $14 filter, but spending $230 on an OEM tank from C-Dan solved my problems for a long time (I hope).

Good Luck

Rocky

 

[CNCFJ40]

Debris partialy clogging fuel pickup

...Today I filled a 5 gallon can of gas and bypassed the tank. I did not seem to have any issue's. What I think is going on is I have junk still in the tank and is clogging the pickup. What I dont get is why only at those speeds. Anybody have any opinions or suggestions? The motor is the F.5 with the Aisan carb. Thanks in advance, Dan.

I had a nearly identical problem. Finally solved it by blowing compressed air through the fuel line back through the tank. My theory was that debris partially obstructing the fuel pickup in the tank would allow enough fuel to pass to meet demand only up to a certain point, beyond which the motor would sputter & die.

My $0.02 worth. Good luck!

 

[Thornton]

Had same issue with sons 73, changed the filter and problem went away.

 

[Rocky_LC]

It would be cool to put in a "switchable "Y" adapter, so you could have two selectable fuel filters. Then youi could change one "on the fly" if it got gummed up.

That's just a band-aid solution, though..... and probably more trouble than just fixing the problem....