Inliners International Forums General Discussion Engines Pontiac OHC Intake on an inline 250

Hello all! This site is amazing. I'm wondering if I can get some advice.

I've got a 67 C10 short box I'm driving and building at the same time. I've completely rebuilt the suspension, installed a T5 trans, rewired just about everything, installed power disc brakes up front, and I'm in the midst of dropping a Holley 390 on a Pontiac OHC intake. Everything has worked brilliant up until now.

It's all together, and idled when I first dropped it on. Now it starts under choke, idles very high, then stalls. (feels like the float bowls draining). Sounds to me like a vacuum leak, but I can't find the leak. I'm still a rookie at most of this stuff.

Let me take you back to where the problems started. The Pontiac bolt holes don't line up with the chev, so I used a gasket as a template, drilled and notched new mounting holes in the manifold. It seems to mesh well with the chev 250 stock exhaust (exhaust heat riser is blocked off).

The carb sits on an edelbrock adapter, and sits 90 degrees to the block so that the primarys match up with the Qjet primary inlets on the intake. Fibre gaskets on top and bottom of the adapter.

The heat riser on the intake looked like a huge spot where a vacuum leak could occur, so I used silicone (permatex gasket maker) to seal it up around the inlet holes. This was in attempt to fix the vacuum leak, but didn't make any difference.

Here's what I've got for hoses:
1)Fuel line extended to wrap around to the Holley
2)Vacuum advance from the distributor to the carb
3)base plate port on the holley plugged
4)PCV from valve cover to brake booster with a "T" running to the intake pcv port.

Cables:
1)Choke is a basic setup that works fine
2) Throttle cable is a Lokar cable running all the way around the carb to make up for the 90 degree position of the carb.

I've RTV'd the pcv grommet, and plugged up the port on the front drivers side of the valve cover.

I've adjusted float bowls, but can't get it to idle long enough to be accurate with adjusting anything.

Everything LOOKS good, but like I said, will only start under choke, idle super high, then stall. Vacuum leak, right?

But where?

I know it's gotta be something basic I've overlooked, or maybe just give up on trying to make the OHC intake work, but I thought I'd ask the experts and it's why I'm here. I can post any requested pics of the setup if it'll help.

If it means ripping out the intake and replacing it with an offy or clifford, I'd gladly take a right hook to the ego to get my truck back on the road.

If anyone has any tips, I'd appriciate any help. I'm totally stumped.

Thanks!

Looks nice!
Sounds liike a vacuum leak,
I use starting fluid to spray where any vacuum leaks could be.

Just be carefull when spraying on hot areas it could possibly catch on fire.

WD-40,carb spray etc works also,but for me starting fluid will increase the idle easier,to find the exhaust leak.

Take off the intake manifold to look for any signs of a leak.

I like to use Hylomar on gaskets,fuel,oil, etc does not effect it.

Is your carb a known working carb?

MBHD

I've got some starting fluid, and I'll give that a go in the morning.

I did purchase the carb used, but it had been rebuilt and met with the seller who showed me the new gaskets and bits he'd replaced during the rebuild. That being said, there could still be a carb problem, but I'm still sold on a vacuum leak somewhere.

What are you using to heat your intake? These engines have a rough time keeping your air/fuel mixture vaporized without a source of heat.

Nice work, by the way. :thumbup:

Those base plate adapters (Holley square pattern to Q-Jet pattern) are notoriously bad for gasket surface, and have almost no contact in a few small areas. I'd "paint" it with silicone temporarily as a test rather than continue to re-start/stall - you can clean it up if you find it. You can even use grease, anything viscose enough to seal a leak but not volatile.
Is the vacuum advance connected to manifold vacuum (below the throttle body) or venturi vacuum (above the throttle body)?
I prefer venturi (ported) vacuum, but disconnect and plug to be sure the advance isn't affecting operation and remove diaphragm leak as a cause.
Are you sure the power brake diaphragm is intact? This will have 2 effects if it's leaking:
1. obviously, it allows air to enter
2. it also triggers the PCV to go to WOT mode, increasing the leak volume
If it's even suspect pinch off the line to the booster.

If this set up has ever backfired, there's a very good chance your powervalve has ruptured. It has an almost an identical symptom as a vacuum leak except your mileage and crispness goes away too.

Good point, the PV may be gone.
However, this doesn't equate to the choke use. A ruptured PV feeds fuel all the time (but cannot feed air, since it's submerged below the float level), and would run rich with the choke off, and eventual stalling would be due to fouled plugs.

I'm going to pop it back off today (when my son goes down for a nap...) and I'll snap a few pics of the PCV set up.

I did drive the truck on two test drives and although it seemed to run really rich, it idled fine. Yes, it did backfire a few times, even igniting the fuel inside the carb.

I'll seal up the gasket surface and see if I can pull the carb apart and inspect the PV.

If that fails, I'll check out the power brake diaphram.

Check if pedal pressure is high, especially after a stop. This means that the diaphragm is bleeding down and leaking vacuum.

Could I plug the hose going towards the brake booster as a means to isolate the problem? Or would there not be enough ventilation to idle properly.

There should be NO flow from the T line to the booster (except while the diaphragm either fills or discharges), so shutting it won't affect any carb operation.

Shouldn't the line from the PCV valve be going to the base of the carb?

Hey Guys, sorry for the late reply. I installed a new power valve, and matched the jets and discharge to the Clifford 390. Installed a new intake/exhaust gasket, and cinched it all up.

Then I believe I found the problem. Gas started spitting out the exhaust port at the exhaust gasket. I just couldn't get a good enough seal with the OHC intake. The bolt holes and mounting flanges differ and require modification even thought the ports line up.

Can this be done? I believe so. Just not by me.

I re-installed the single barrell stock manifold, and it went together so nicely that I almost felt bad for separating everything.

It also made me realize that there are intakes out there that do a really good job like the Offy or Cliffords, and the price is worth more than the hassle for your regular driveway mechanic.

The carb is ready, and I'm saving for the intake... Lesson learned.

Do you mean the thickness of the flantges differ?

If so I've read several posts here of people shimming up the tie bar on the thinner one.

Hope that helps.

Here is one on Ebay
http://cgi.ebay.com/ebaymotors/ws/eBayIS...RTQ:MOTORS:1123
This is what he said about it.


MBHD


the man I got it from ran it on a chevy 230. I ran it on a chevy 292 c-60 1966 dump truck for over a year because the one barrel would not stop leaking. It was rebuild 2 times so I changed it over to a 4 barrel. I did not run the exhaust because the back exhaust pipe was right at the linkage for the clutch. If you look close at the picture you can see the bolt that was a through hole to plug off the hole so I could run the stock exhaust manifold. It worked very well,the man I bought it from bought it from a local machine shop.

I do not think the Sprint cyl head will flow much better than a nicely setup siamesed port Chevy head.
Although I have not looked at a Sprint cyl head in years.

The real main adavantage is the ohc,should be able to rev high.


MBHD

I agree, I don't think it would offer any advantages over the siamese head for the work involved to do the swap.

It is a suprise that the intake worked at all. The head on the Pontiac is about 1 inch longer. It does share bore spacing and size, plus you can put a crank from a 292 in it, but most other parts are OHC specific. The complete gasket set for a Chevy 250 will cost you about $35.00 but a set for an OHC will cost adout $115.00 ( 6 years ago) and it will probably have to be ordered.

firebirdst
1969 Sprint in a 1962 Nova 4 door

firebirdst,,
lets see some pics of your Sprint eng in your 4 dr Nova.
Sounds cool.


MBHD

I have some ready to go, how do you post them here?

Firebirdst

as you can probably guess I have owned this engine since November 1968!

You can use photobucket.com,or others.

MBHD