Hesitation Issue and Valves

[dotgain]

Hi Guys need some help…

I have all the mechanical done on my truck and have temporarily put the cab on to work out any bugs prior to finishing the bodywork and wiring.

Everything is working well… just like shes fresh out of the crate, except that I have a slight hesitation in the engine speed coming off idle. I would guess around 1500 rpm.
You can work through it easily, seems to be a very narrow rpm range that has the issue.
Motor runs strong otherwise… lots of power and smooth at all other rpms.
If you hit the gas hard when at the stumbling rpm she sometimes pops out the intake.

Here is what I have done (my terminology may be off a bit)…
pulled carb, cleaned and ensured all passages are open, replaced accelerator pump piston with one in better shape, it pushes gas well into carb manually.
I have stretched the spring slightly (about .25'') on the intermediate circuit? to help lift it off the seat, no real visible wear on the needle. holds a tight seal to the seat/valve.
reassembled with same result.

I have tried multiple timing settings, seems to run best around 5-10 degrees ATDC. however hesitation still there.

Mixture set to highest vacuum at idle and where it runs smooth.

This is a CDN 251 truck… I am now setting valve gaps and manual says IN @ 0.010 and EX @ 0.018 hot.
The motor was rebuilt by a previous owner who had the cam/valves/tappets/pistens/rings/bearings all in, I know the cylinder walls were done up and new bearings/rings.
The valve seats were done but not sure if the valves are new or had the ends resurfaced? They looked to be in good condition prior to assembly.

Set the governor as per manual.

I have read in a thread where Charles mentioned a IN 0.009 and EX 0.011 hot on flat tappet screws… would this apply to the 251 motor as well?
Seems a long way out from 0.018 on the exhaust.

If I do not find anything wrong with the valve gaps/lash then I am thinking timing advance issues with the distributor?

Any help/advise would be appreciated.

Paul.

[w30bob]

Hi Paul,

I'm sure the "crew" will be on here to help you out in a short bit....but the one thing that caught my eye was your timing numbers. You did mean 5-10 BTDC, not ATDC.....right? No engine should run good with ignition after TDC.

regards,
bob

[RMS]

You did mean 5-10 BTDC, not ATDC.....right? No engine should run good with ignition after TDC.

the canadian eme says 2 atdc.

Paul i have had the same slight hesitation on my 251 for years on a complete rebuild. the carb was even sent out and a custom kit made as I could not find one. (the 251 jets are larger than the 230s). I messed around for years and the only thing i could come up with is lag from the change over from the low speed circuit to the high speed circuit. as the bearing rolls between the circuits it jams a little causing the hesitation. if you add a little choke you may find that the hesitation is not as pronounced. I also found adding some Seafoam to the gas tank made the off idle hesitation less pronounced. you may also find that you lose throttle control coming down steep grades a quick blip of the choke will get the gas flowing again.

[dotgain]

5-10 BTDC correct my error.
I Was speaking with the father inlaw and he wondered about a small fiber washer maybe 0.010 thick at the bottom of the bore on the low speed circuit to allow a bit more fuel in when under heavy vacuum?
Paul.

[dotgain]

I read in another thread that the 2 ATDC is an error in the manual, I assumed this applies to the 251 as well.
Runs best somewhere around 5-10 BTDC.

[w30bob]

Hi dotgain,

That's one heck of an error to put in a manual! I guess those in the motorpool would have spotted that one right away......or those trucks tuned to that spec would make about 10 hp on a good day!

regards,
bob

[topellis]

Sounds like you are making lots of progress. The only thing I thought to ask about is whether or not tinkering with slight choke settings makes any difference. Seems like I had a similar problem after I rebuilt my carb and I had the choke slightly out of adjustment.

Good lunch
Mark