Technical Pulling my hair out, engine stumbles under load.

Idk what the flathead should show in a comp test but that's pretty damn low. What's the compression ratio on those things supposed to be?

7:1 should be around 140 ish

 

7.5 should be in the neighborhood of 120 , 8 - 140 and depends on cam overlap

 

So what's 110 equate to ?
And what's it supposed to be for 37-38 flat head?

 

Been looking at it, seems my compression test shows all to be pretty much in the ballpark. Compression ratios on these are like 7-8:1 so I think I am OK. The fact they are so close to each other is the other aspect.

http://www.flatheadv8.org/flatratio.htm

110 equates to a 7.3:1 ratio. As far as I know the multiplier is @15 although I started reading this and then wished I hadn't....

http://en.wikipedia.org/wiki/Compression_ratio

I will do a leakdown test which will help but in all honesty with readings this close together the issue lies elsewhere. Carb rebuild kits and 4.5PVs arrive today courtesy of UPS. The sooty plugs pretty much tell me what the issue is.

 

You really want to diagnose your power valves with a vacuum gauge. It should be half of your total vacuum at cruising speed. Also want to tune your idle for the highest possible vacuum.

 

I see no mechanical fuel pump, must be electric, usually they need a regulator for these old carbs, although too much pressure should show up at idle, is there a regulator somewhere we can't see, maybe it's too restrictive . Also try pulling the choke out and see if it helps or hinders....if it helps your too lean ,it hinders your rich


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I wouldn't go over 6psi on that application. It can over power the needle and seat and flood. Can you see fuel leaking into the bowls at idle?

 

From what I have read fuel pressure should be like 2-3 PSI, anything more can causing flooding. I do have a regulator but am running blind. The fuel pressure gauge arrived with the vacuum gauge so I will know more this weekend when I hook it up. Will be aiming for like 2.5 pounds of pressure.

Again this weekend I should know a lot more. New plugs are in situ, just need to rebuild the carbs, replace the jets and PVs and run it to see what vacuum I have.

 

Wow 2-3psi? That's like urinating.. I wish you the best, I chased my ass with my power valves. If your regulator is like mine, turn it out to lower the pressure. Good luck. I have a holley blue pump and regulator.

 

Wish I could pee at three psi ! The pressure a carb wants is determined by float size length of arm from float to pivot etc.you don't need a lot of pressure but you need sufficient volume. More volume under load than in the shop sittin still so try to check your pressure when driving......just a thought but if there is a restriction (too small I.d. Fitting for example you could be rich at idle and starving under load) also if you hook up a timing light and tape it to the windshield you can watch your "spark" while driving .good luck and tell us how you make out!


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Lmao

 

Maybe It's not pressure but fuel flow. Look for a minimum of 1.5 to 2 qt per min.

 

have your doc install a Holley red ...

 

Life kinda got in the way of my spending a lot of time on the car but I did get the carbs back together with 49 jets and 4.5 PVs. The car runs a LOT better, almost no stumble at all now.

I need to get them tuned. I have a uni syn from tuning the wife's old MGA with SUs so will use that. One carb is dripping from the spray bar. I pulled the top and rechecked the float level which was fine. Still drips, I will need to get to the bottom of that.

Vacuum is erratic and in the 15-17 region, moving quickly between the two.

Once odd thing I noticed, the idle mixture screws on the rear carb, one screw when all the way in causes a reduction in idle speed. The other has zero effect when screwed all the way in?

Anyway, like I say this is a BIG improvement. The stumble is all but gone now. I will run it for a while and see how the plugs look. Might be a case of fine running the PVs/jets as I get a feel for how she runs.

Thanks for everyone's input along the way.