Installing a 67 tri power intake manifold

[Dave C.]

I am just about to install the intake on my L71, and like a good boy , I have been searching the archives before asking this question.after reading a few posts about the restrictor plates that you can install in the centre exhaust port, it seems to me that the idea that makes the most sense is to use one supplied with the hole in it on the choke side and completely block off the other side ( drivers side) I guess you would still have to open up the heat riser some how. Or maybe it would be better to make block off plates with smaller holes? So you still would get some flow through. What I really don't want is the same hot starting problems I had with the last tri power. I was also thinking that if you did block off the drivers side completely, could you wire the heat riser open just slightly, to get more heat up to the choke, and then it would still open fully when up to temp ?

[John M.]

Re: Installing a 67 tri power intake manifold

Hi Dave,
I wouldn't be frightened too much by the hot starting issues. Mind you I live in Vermont but I have been driving a stock 69 435 Tripower for 16 years and the only hot start issue I ever had was the starter itself. I got stuck idling in a 4th of July parade on a very hot day and was just waiting for something bad to happen and it never did. I may have spilled some anti freeze but could not say for sure; the car behaved extremely well. The Holley carbs, of course, leak every spring for a while and I have flooded the car but it always starts.
Maybe I'm just lucky.

[Ken A.]

Re: Installing a 67 tri power intake manifold

Back in the day we poured melted, used pistons into the head. All those hi po gaskets wound up burned thru fairly quick. You need to block the head side.

[Duke W.]

Re: Installing a 67 tri power intake manifold

As I've suggested many times, I suggest you block he driver's side competely with a shim with no hole. Leave the passenger side heat riser passage completely open. This will form a "dead end" passage that should allow enough exhaust heat to reach the choke stove to allow normal choke operation, but since the passage is blocked, which won't allow side to side flow, the amount of heat introduced to the carburetors should considerably ameliorate hot starting issues from excess vaporization or percolation from the fuel bowls.

Make the shim from .010" thick 304 stainless steel stock, and place it between the inlet manifold flange and the gasket, and leave a tab on the shim that you can grab with a set of neele nose pliars. In case you want to remove the shim, simply loosen the inlet manifold bolts, extract the shim, and retorque the bolts.

It is ABSOLUTELY NECESSARY AND CRITCAL to securely wire the heat riser valve full open. Use stainless steel safety wire. If the heat riser valve closes the RH exhaust will be completely plugged!

Also, understand that the exhaust note will be different - like the difference between the 365 HP and 375 HP FI engines because like the FI engine there will no longer be a "crossover" pipe between the two banks as there is on the 365 HP carbureted engine.

Duke

[Dave C.]

Re: Installing a 67 tri power intake manifold

Thanks guys, Duke, I just picked up a new set of gaskets since the machine shop I went to to have the intake shaved a bit , told me the ones I had are for a cast iron intake, so the new blue Felpro ones are completely blocked off, is that enough to block off the drivers side and then just cut a hole in the choke side? And when you say the exhaust note will be different, do you mean better or worse?

[Steve G.]

Re: Installing a 67 tri power intake manifold

There is controversy around this topic and I appear to take the unpopular side. But I think that in as much as I see that you also live north of the 49th, I will put it out there. Hopefully this won't upset anyone.

The heat riser/exhaust crossover "fix" has been identified as the remedy for fuel "percolation" causing hard hot starts on hot days. This does not mean hot engine starts on any old day. It is primarily influenced by the temp of the fuel leaving the tank. Perc has been with us a hundred years. It is said that in the US where all of their fuel has about a 10% ethanol content, the problem manifests itself at a lower temp than it used to.

If you go to a service manual and look at the fuel system diagnostic charts there are at least a dozen check for hard hot start. If you didn't go through all those when you had your problem you probably shouldn't assume that perc is your problem. Perc can be identified by plentiful white "steam" coming from the bowl vents after shutdown. Hard hot starts are often misdiagnosed as fuel perc. Two very recently right here.

We have different fuel.

We live in a different climate. I see you're in BC. I'm in Kamloops. As you likely know, we get hot summers. I have never experienced fuel perc here driving my 67 442 around here on a hot summer day.

So what's the down side of altering heat riser and it's associated parts. Depends on your definition of a proper running car. Mine is being able to start the engine with no more than 1 or 2 pumps of the pedal. As soon as it's running I should be able to kick the pedal once to knock it off the top fast idle cam, put it in gear and drive away. There should be no coughing, wheezing, spitting, sputtering or stalling at all right from the time I start it right through to fully warmed up. I should be able to do this with a cold engine from -25F to +100 F. I should be able to shut it off at any point of it's warm up and restart it without issue. Likewise fully warmed up.

If you tie your heat riser open and block one side of the exhaust crossover, you will not be able to accomplish the above. If you don't care about how it performs during warm up or on a cool fall day, no need to read any further.

With the heat riser wired open and one or both sides blocked, if you do not significantly alter your choke system, your choke will seldom, if ever, come off fully. If it is not coming off fully and/or taking too long to do so you will go through fuel like s... through a goose. If you shut it off after it has warmed sufficiently to not need a choke for starting, and try to restart it, you will encounter a hard start condition. It is choking a warm engine.

There is a very limited amount of adjustment available to you in a manifold well style choke. Additionally, when you reduce choke tension sufficiently that it comes off hot, it will not be coming back on at the temp it needs to for a cold start.

Others report using an electric choke after having done this mod. You will need to look at that or a manual choke.

There's more to the function of the heat riser as well, best we not go into it here, but search it in the "Gasoline distillation chart" thread and the "heat soak" thread.

In my opinion only, it is sacrificing a lot to try and eliminate a problem that sounds like it was never confirmed as being your previous issue in the first place, one that's climate related and exhibits only under conditions you will see infrequently and a problem that isn't being found to be occurring in the area near you.

This opinion formed does not come from reading internet articles. I worked in Winnipeg as an Interprovincial Journey mechanic in the 70's and 80's when carbs and chokes were all we had. Hard hot start, high fuel consumption, engine sluggish unless wot were common summer time complaints. Real problem with Chrysler product small blocks, 318 and 360. Very high rate of heat riser failures. If left long enough the bit of exhaust that made it into the crossover formed hard carbon and plugged the passage completely. After repairing heat riser still no heat you had to remove the intake and chisel out the carbon in the crossover. Didn't happen as often with GM because they had few heat riser failures.

And every fall I would be inundated with cold engine driveability complaints. Most people don't like to have to sit and wait for their engine to warm up before they can move it without stalling.

FWIW.
Steve

[Duke W.]

Re: Installing a 67 tri power intake manifold

Thanks guys, Duke, I just picked up a new set of gaskets since the machine shop I went to to have the intake shaved a bit , told me the ones I had are for a cast iron intake, so the new blue Felpro ones are completely blocked off, is that enough to block off the drivers side and then just cut a hole in the choke side? And when you say the exhaust note will be different, do you mean better or worse?

You didn't say whether it's a 400 or 435 HP engine, but they have different port cross sections, so you need an appropriate gasket set.

Why don't you find a correct set of OE type gaskets and go the shim route?

Otherwise, I recommend you cut out a full size heat riser opening on the driver's side gasket - equal to the opening of the heat riser passage. Leave the driver's side alone, but that eliminates the easy way to open it up by removing a shim if it doesn't work out. When you fire up the engine remove the air cleaner cover after it's at full operating temperature and observe if the choke valve is fully open.

If you don't typically do cold starts below garage temperature, you probably won't notice any drive-off problems. At worst, you might have a bit of start-off stumble for the first few minutes of operation, but hot starting and general hot running behavior should be better.

If you go the shim route, wipe the side against the gasket with silicone spray lube so the gasket doesn't stick in case you decide to pull it out.

I can't tell you if the exhaust note will sound better or worse with the heat riser blocked to your ears. You would have to compare side by side a 365 HP with an open heat riser to a 375 HP FI engine, both with the exact same muffler types.

IMO the FI engines and carbureted engines with blocked heat risers sound more "gutteral". There's more low frequency sound coming through each bank. I think most guys would like it or would say it's okay or get used to it if they didn't like it. It's a minor consideration IMO if the engine exhibits better hot operation in summer temperature around town driving. Dealing with poor low speed operation and hard hot starting due to excess fuel vapoization and percolation is no fun.

Duke

[Dave C.]

Re: Installing a 67 tri power intake manifold

Steve, Duke thanks so much for your imput, Duke , I did mention that it was an L71, but it's so far back now you probably didn't notice, and Steve I actually live south of the 49th , I'm in Victoria lol, I think I did have purculation problems with the L 68 that was in this car when I bought it as I did have a lot of steaming from carbs , not sure what to do now, I would love it , if I could just cut the centre wide open, leave the heat riser alone , and have it run perfectly in the summer and winter , but after going through the hot soak problems I had with the 400 horse , anything I could do to help that problem would be the way I would lean. Nothing like stopping for a few minutes somewhere and having everybody standing around your corvette as you give it half throttle and crank it over for 20 seconds until it starts.

[Steve G.]

Re: Installing a 67 tri power intake manifold

Steve, Duke thanks so much for your imput, Duke , I did mention that it was an L71, but it's so far back now you probably didn't notice, and Steve I actually live south of the 49th , I'm in Victoria lol, I think I did have purculation problems with the L 68 that was in this car when I bought it as I did have a lot of steaming from carbs , not sure what to do now, I would love it , if I could just cut the centre wide open, leave the heat riser alone , and have it run perfectly in the summer and winter , but after going through the hot soak problems I had with the 400 horse , anything I could do to help that problem would be the way I would lean. Nothing like stopping for a few minutes somewhere and having everybody standing around your corvette as you give it half throttle and crank it over for 20 seconds until it starts.

Dave,

Well, if you're in fact south of the 49th, that changes everything.

Don't confuse steam coming up the throat with steam from the vents. If the carb is dripping internally the fuel will vapourize in the throat and produce visible vapour too. Do you have any recollection of the kind of day when you saw that? It would be what Islanders would consider a blistering hot day. And would only do it on those blistering hot days if it's fuel perc.

Pretty hard to diagnose at this late stage, but given how often this seems to be misdiagnosed, the typical summer weather in Vic, and the fact that our gas in Kamloops doesn't boil here in the summer, I'd be reluctant to bet the farm on boiling fuel.

I also don't think that John McRae's Vermont summers are much different from us. And he's got the lousy gas.

I would want to do some research into what your options for a replacement or alternative to your choke stat are and how important appearances under the hood are. You may not want to find out after you have this together and are sorting out the choke problems that the only solution is a cable.

If you're up this way get hold of me. There's a 67 L71 and 69 L36 in my shop here.

Steve

[Dave C.]

Re: Installing a 67 tri power intake manifold

Thanks again Steve , not sure which way to go, that L68 was an older motor, who knows all the problems it might have had. I thought this would be more cut and dried, everybody, including local people have a different opinion on this one, my previous 66 L72, never had any issues, but I
never got inside the motor to see how it was configured. I get my intake back tomorrow, so I guess I will figure something out by then. Thanks everybody for their food for thought.