Abrupt power loss at 5.5k on flat and uphill. 1969 srl311

[SLOroadster]

This is starting to sound a little bit like the 1600 race car I drive from time to time. I'm thinking its not your valve springs, although from the looks of those adjusters your valve seats are pretty well shot (Not totally dead, but not far off.)

Your plugs do look lean, and since pulling the chokes open didn't seem to fix anything, perhaps its worth a look at the float bowls, you might have a sunken float(s). Also, check the hose that runs from the float bowl to the jet. Also, perhaps you have a vacuum leak that for some reason only makes itself known at high rpm. (How are your throttle shafts for play?)

Lastly, I wonder if your fuel pump simply is tired and can't feed enough volume at high rpm? It could also be the lobe on the jackshaft is worn to the point that it doesn't pump the fuel pump enough to support the fuel flow needs of the engine.

Will

[fj20spl311]

Try a small gap in the plugs 0.030-.032

[jhayden]

Chris,

If you’ve mentioned compression readings since your rebuild, please forgive my oversight. If you’re aiming for 7K, the readings need to be near normal and the hi-lo spread negligible.

The stock fuel pump is rated for 3.4 to 4.3 lb/sq.in @ 700 rpm. If in doubt about yours, you can pick up an electric for very little money and substitute as either a temporary (if you wish to keep the original) or a permanent fix.

Jon

[1969311]

Ok, this is good news, thank you for taking the time to look. So many rods in the fire:)

Had to remodel the garage today so I could fit my wife's first restoration in, now that we're back in a semblance of order, I can wrench again:)

I will really be pleased if some extra eyes in shasta can assist me in a direction. Forums are great for so much, but so often when an expert has a look something sticks out to them as plain and obvious. That is assuming I am unable to sort this out before then!

Thanks again!

[1969311]

Sorry I missed a few of the other replys,

Will: I had the carbs redone by z therapy about two years ago, the throttle shafts have little excess movement side to side. The floats are the hinges types, and appear to be intact with the lever actuating at the proper angle. Both of the fuel tubes are supple with no obvious kinks or restrictions.

I will dig out my vaccum gauge and do some diagnostics at different rpms.

I'm almost certain my uncle replaced the jackshaft in the rebuild, will look through the receipts, but I like the thinking
I have two other fuel pumps that came with the car, condition unknown, and was considering picking up a rebuild kit, though I should probably plumb in a pressure gauge to test the flow output before jumping to that. I like the electronic pump idea for a temporary test. I admit I am partial to the original equipment, but not when it's reducing performance!


Fj20spl311: I will try a smaller gap and see if that helps, simple test that is easy to control variables with .

Jon: I have yet to do a compression test as I was hoping to properly seat the rings, I will do a test and get some numbers for the books. Thank you all for the insights, I will conduct some tests and report back accordingly!

[Tomc18]

Looks like you have tried everything - so here is one more thing, which just came up in conversation over here in Aus

When you had the carbs rebuilt, did you hard mount the carbs? Did Z Therapy hard mount the float bowls?
there's a resonance point at 5-5500 that shakes the fuel so much it doesn't flow unless something is soft... good luck!

Tom

[1969311]

Tom,

Forgive my misunderstanding, what do you mean by hard mount? As in metal from the su direct to the intake manifold?

I have the stock hard rubber insulators between the carbs and the manifold, is this what your referring to?

Thanks for the help!

[redroadster]

We have just insane humidity here in KC. and that will do what you describe .my enduro bike I ride around on had a stumble at mid rev slow to reach high rev , yesterday with low humidity it fixed its self

[Tomc18]

If you have the rubber insulators you should be ok, if you swap to the harder bakelite or alloy spacers then it gets bad vibration

later SU's had the float bowls between the carbs very softly mounted, you can tell by pushing on them as they will move around, which helped solve the problem

[1969311]

Yes, mine do move around, thank you for the clarification. I am going to do a compression check and get some numbers to compare.

Thanks again, I appreciate it very much!