Hello everyone, I'm new here so my apologies if this one has been beaten to death already. I've looked around the forum and found a few possible fixes but hoping you guys can help narrow it down. If my Rocket 350 sits for 7 or more hours, you need to dump some gas down the intake to get it to fire. You can pump away till you're blue in the face or crank until the battery dies, nothin. Once you pour gas in, it will run like a top. I've changed the fuel filter to the correct one with a check, plus I have a new carb coming. The fuel pump is delivering fuel fine once its running so I'm not suspecting any issues there. There are no leaks that i can see in fuel lines either. Any suggestions?

 

Check for two streams in the primary throttle bores when carb is opened. You need two streams, otherwise the accelerator pump is likely bad.

 

Thanks a lot, I'll check that after work.

 

I had a 72 Cutlass Supreme with a similar issue - if it sat for a few days you needed to crank it for a minute or so until fuel finally worked its way up…rebuilt carb and checked all the usual suspects. Turned out the next owner diagnosed that one of the rubber lines (forget which one) coming from the fuel tank was cracked…I forget the reason this caused the issue but he told me it was a relatively easy fix and resolved the problem. Good luck….

 

Worth checking that too. I didnt think about that being an issue since I've never seen gas on the garage floor.

 

Same issue here, accelerator pump was shot.

 

Would that also manifest itself when driving?

I have the same issue in my '71 (350 with QJet), stumbles when I accelerate without feathering the pedal a bit which I've assumed is a bad accelerator pump. Just haven't got around to dealing with it.

 

I'd think the line between the pump and the tank is a negative pressure (vacuum) rather than a positive pressure that would push fuel out of any cracks or pinholes. Depending on the location and size of the hole, the vacuum would probably cause outside air to seep into the gas line when running. From the cold start, the hole would prevent the pump from creating enough suction to get the gas up the line until the engine's RPM is activating it fast enough to overcome the leak.

Just speculating, but that's what sounds logical to me. If the pinhole/crack is on the top of the gas line, you're less likely to find any leakage. There's probably a relatively easy way to test for leakdown in the line?

 

I would think that after 7 hours there would still be fuel in the bowl.

 

Yes but only after I reduced the idle. I think before I got it, they ramped up the idle screws to mask what is appearing more and more to be an accelerator pump issue, at least in regards to throttle response. The seller’s friend was the one who suggested it could be the wrong filter, which it didn’t help but needed to be replaced anyways. After reducing idle, it sputtered and nearly stalled as I was trying to take a left off a main road. I could replicate it my garage by giving it a fast rev since it wasn’t under load.

I also may have to replace fuel lines anyways. Just to rule that out if the refurb carb doesn’t fix it. That’s the only other thing is could be.

On a side note Vacuum is around 17” WC, I’m reading some conflicting reports on what’s optimal.

So far, thanks for the quick responses everyone. Really appreciate it.

 

Steve, think about what Eric's reply in post 9 means....

It means that the starting problem is 100% in the carb. The carb would also be the source of the other problems you have described.

There may be a separate issue in the fuel line, but that is unlikely and it would be unconnected to the problems you have described.

You'll be money and time ahead if you stay with the carb problem and don't escalate.

Gary

 

Just my opinion, but buying a whole 'nother carb when the problem may simply be a worn accelerator pump seems a bit extreme.

 

You’re not wrong. I’m used to Motorcycle Carbs so I shoulda got on the forums sooner. On a plus side it was only $200CAD for the refurb.

I’ll see if I can cancel the order and go the parts route. Thanks everyone I’ll give an update in a few weeks to resolve the thread.

 

Cheap rebuilds can easily bring in a fresh set of problems. Even if you can't cancel your purchase, do as Kenneth intimates, rebuild your existing carb.

 

Soooooo,
1. Did you confirm two streams of fuel in the carb primary bores when pulling open the throttle ? Or not ?
2. What idle SCREWS are you referring to ? Idle mixture screws ?
3. Reduce the idle with WHAT SCREW/SCREWS ?
4. Fuel lines can be checked AFTER you checked out the carburetor.
5. What does 17" WC vacuum have to do with anything ? That sounds like engine manifold vacuum which has nothing to do with vacuum pressure in fuel lines.
6. Your fuel pump should supply fuel to the carb at about 5 PSI

 

X 3

What kind of motorcycle do you ride ? I have never seen a motorcycle carb that couldn't be rebuilt.

 

Well I had rode Harley’s for years but started rebuilding 70 era Japanese enduro’s. Yes those carbs were rebuilt but are a lot closer to a snow blower than a car. I finally have the means for car projects, at 33 years old learning older systems takes time and I’m finally digging into this one. My dad suggested starting fresh, personally I’m a rebuild person but I’m just getting into older cars.

1. No I haven’t, big snow dump dealing with that.
2. Idle screws only. Choke was set way too high and idle
3. Idle
4. absolutely agree
5. This is just added information. The vacuum was measured at intake pressure. Some local boys asked what my manifold intake vacuum was to check for leaks first. This was more auxiliary info and I agree should have no direct affect on this problem. But being it’s the next measurable info after the carb I wanted to rule that out. I’m still not sure if that’s satisfactory vacuum since I’m reading conflicting information. Figured here with a ton of old’s engines I could get a baseline or even better, a spec. The needle is steady when reading too so that’s a positive.
6. I will check that next. But I don’t see how my issues would be cause by lack of fuel after 7 hours of engine off. Engine off is 0PSI. The question is why.

 

The older carbs were simpler, usually idle circuit, pilot jet and main jet. The only things that needed was needle and seat and throttle slides. Harley's were a little different, but I never heard of people buying a carb rather than rebuilding them. I have one Harley...1977 XLCR, a BMW R/50, Yamaha AT1 and a bunch of BSA and Triumph's. All the carbs are rebuildible. I never heard of buying a new because the old one needed rebuilt.
The idle screws you are referring to are probably idle mixture screws. They are to optimize the fuel/air mixture at idle, not to set idle speed. There is ONE screw that is used to set idle speed.
Speculation: After 7 hours, all the fuel vapors in the intake manifold and cylinder head runners have evaporated. So, a quick press of the gas peddle shoots a little fuel in the carb bores and it starts. If the accelerator pump is bad, you get no squirt of fuel. You can fiddle around with everything else and nothing gets better.

 

Thanks Ralph. My focus will be the Accelerator pump. Fingers crossed it’s the fix with a bit of tuning.

 

It was the accelerator pump. Also, the choke was not set properly so the tab blocking the secondary's would never retract, it was definitely worth ruling out the Carb first and avoiding a snowball of unnecessary work. I had time to tackle this yesterday so I apologize for the delay. It fired up right away this morning without needing to dump fuel down the carb. Also, zero hesitation or sputtering under quick throttle input. Thanks Gary and everyone else.

Next item on the list, Rocker Arm Adjustment. She will be running right before spring.

 

Glad you got it fixed. Stock rocker arms are fixed, no adjustments.

 

The KISS method is always the best troubleshooting advice.
FYI unless someone installed aftermarket adjustable rockers or push rods there is nothing "adjustable" under the rocker covers on a stock 350 like there is on an Old School SB Chevy.
If you hear rocker noise you need to replace the worn parts (which will lead to MAW$). Make sure you are not hearing an exhaust manifold/head pipe leak.

You could get temporarily lucky replacing one or two trunnions (pivots) and rocker sets. But if it's making a lot of noise it could be deeper or more than just R&R-ing a few arms and pivots. Inspect the wear points on the pivots and rockers. Make sure the exact same parts go back in the exact same spot if you reinstall the used parts to include push rod ends. You will want to roll the pushrods to check for straightness.

https://www.summitracing.com/parts/c...yABEgIjnvD_BwE

 

Steve, congratulations on getting this fixed.

AND realize that you are in a minority, in that you take advice and run with it instead of pursuing your own agenda regardless of feedback. Congratulations on that as well.

 

Oooph. Thanks for saving me the time and preparing me for the cost. I’ll browse the forum for similar topics. Thank you.