aussiecowboy Posted March 29, 2012 Share Posted March 29, 2012 Greetings from Australia. I drive this car a lot, just not long finished a 2000 mile trip through the desert with it.For some reason now though the engine won't rev out cleanly, gets to maybe 2000rpm and it's like it hits a govenor. I thought it was the carb (original updraft) leaning out initially, I had some spare manifolds which I modified and changed to a stromberg downdraught. I also rebuilt the mechanical fuel pump and added an electric boost pump. It started easier and idled smoother but the same problem remains.I then moved to the electrics and changed from 6v to 12v, again better starting and idling but problem remains. I suspected a burnt valve or broken valve spring for a bit but generally that presents as a miss at idle and disappears higher in the revs as the engine is moving faster than the valve can leak. I need to make a fitting to do a compression test, the plugs are too big for all my fittings. So, timing was next on my list, I cleaned up and lubed the mechanical advance mechanism, no difference. I then held the engine wide open against the flat spot and moved the distributor either way until it started to die but the flat spot remains so it can't really be timing. It runs beautifully otherwise, responsive to throttle until that point then it just makes a flat barking noise back through the carb. I put an adjustable jet in the new carb and have tried all sorts of tunes from very lean to almost blowing fuel out of the exhaust.I'm now starting to think possibly softened valve springs leading to valve bounce? An internal earth fault in the distributor? Other than that I'm lost. I would be grateful for any suggestions.Heath Link to comment Share on other sites More sharing options...
Bill Harmatuk Posted March 29, 2012 Share Posted March 29, 2012 I would do a compression check on each cylinder.If the motor pops back through the carburetor.... It is a symptom of a burnt valve.Good luck.Bill H Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 29, 2012 Author Share Posted March 29, 2012 That was one of my thoughts too but doesn't that usually get less noticeable at higher revs?Compression test a must, just as soon as I find a fitting that will screw into the massive plug holes. Link to comment Share on other sites More sharing options...
Grimy Posted March 29, 2012 Share Posted March 29, 2012 Making a fitting for the 7/8 spark plug holes: Find an old (junk) take-apart-for-cleaning 7/8 spark plug and do indeed take it apart. Then thread the interior of the base for 14mm, the standard thread for compression testers. The exterior of the base has the 7/8 threads that screw into your plug holes. Link to comment Share on other sites More sharing options...
nzcarnerd Posted March 29, 2012 Share Posted March 29, 2012 While you are on the go you could buy some plug adaptors which a neighbour of mine supplies here in NZ ($9 each) and change over to much cheaper, modern plugs, as seen here - About Quiz #316: 1926 Pontiac 6-27 (Upd. two sons, two carbies) - PreWarCar - on my sons' toy(check the link to the engine pic). Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 29, 2012 Author Share Posted March 29, 2012 Anyone what the thread pitch is for the 7/8 plug? I'll turn an adapter up on the lathe here at work. Link to comment Share on other sites More sharing options...
Grimy Posted March 30, 2012 Share Posted March 30, 2012 18 tpi, as I recall. To confirm, put a thread gauge on a regular 7/8 plug. Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 30, 2012 Author Share Posted March 30, 2012 Thanks. I can't confirm on a plug as I am currently on an oil rig off the coast of India, some 5500 miles from my beloved Pontiac. Link to comment Share on other sites More sharing options...
Dave Mellor NJ Posted March 30, 2012 Share Posted March 30, 2012 I believe a 7/8 plug is standard 1/2"pipe thread(NPT in the states). It is the size of Ford model T while the Model A uses 1-1/8" or 3/4"pipe thread(NPT). You can probably cut off a piece of 1/2" rigid pipe and use your tester right inside it. Link to comment Share on other sites More sharing options...
Dandy Dave Posted March 30, 2012 Share Posted March 30, 2012 I once had a similar problem with an old IHC Scout. Turned out to be a weak points spring and the points would float at high RPM's Dandy Dave! Link to comment Share on other sites More sharing options...
Guest cben09 Posted March 30, 2012 Share Posted March 30, 2012 Agree with Dave,,A weak point spring will allow the points to stay open while the cam goes round and isn't making contact,,hence the skip,,A rev limiter in disguise,,, 7/8"-18 and 1/2" pipe were two early plug threads commonly used,,Cheers,,Ben Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 30, 2012 Author Share Posted March 30, 2012 A nice idea but unfortunately one that I also had. I have put two brand new sets of points in the car with no difference. I have a pertronix kit on the way to eliminate it alogether.The more I think about it the more I reckon the valve springs have lost their strength. Link to comment Share on other sites More sharing options...
Grimy Posted March 30, 2012 Share Posted March 30, 2012 This is strictly a long shot but can be quickly checked. I had a similar but much worse problem (apparent at 800 rpm under load) a few years ago with my 1925 Pierce Series 80 coupe. After two of us spent a lot of time unsuccessfully trying to diagnose, we finally discovered that on that car, unlike most others, the vertical pivot post upon which the ignition points are mounted is **not** a press fit into the dist plate, but rather is secured on the underside of the plate by a nut and lock washer. The lock washer had fatigued from age, allowing the nut to loosen, with the effect that the pivot post would rock almost imperceptibly above idle rpm under the point spring's load. That skewed the points' performance substantially.To test, remove points from the pivot post and try to rock the pivot post. If the post does not move, that's not the cause, If the post moves at all, then remove the plate and inspect its underside. If you do have a nut and lock washer on the underside, replace the lock washer and re-tighten. Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 30, 2012 Author Share Posted March 30, 2012 A good thought, and one I hadn't considered previously. I'm fairly sure that the post is a press fit but that doesn't necessarily mean it isn't moving. Worth checking! Link to comment Share on other sites More sharing options...
Guest Foggy norm Posted March 31, 2012 Share Posted March 31, 2012 Consider your fuel supply. I drove a large truck, which another driver lost the gas cap, then shoved a rag in the opening ( I know what your thinking). I drove that truck all day and noticed the rag had disappeared, I made a cap from aluminium foil to get through the day. A week later the truck began acting-up with similar symptom's as you mentioned, had power but acted like a govenor kicked in. The company mechanic discovered the missing rag had fallen into the tank and dissolved/disinergrated, clinging to the siphon. It was allowing enough fuel for the engine to work, but enough restriction to hinder the torque. You mentioned driving in the desert...sand where you don't need it? A collapsing exhaust pipe would show similar symptom's. Link to comment Share on other sites More sharing options...
aussiecowboy Posted March 31, 2012 Author Share Posted March 31, 2012 ha ha, rag speed limiting, I love it. Sadly though I don't think it's fuel, I have drained and cleaned the tank and blown the lines back through. I also run a transparent disposable fuel filter in the line which is still spotless (I changed it anyway). The exhaust system is brand new front to back. For the trip through the desert I made sure I had a good sealing vented fuel cap which I wiped off before opening. Link to comment Share on other sites More sharing options...
ch1929 Posted March 31, 2012 Share Posted March 31, 2012 Even though the exhaust is new I would drop the front pipe and see if it revs. I have seen mufflers with internal blockage cause that problem.Roland Link to comment Share on other sites More sharing options...
Dandy Dave Posted March 31, 2012 Share Posted March 31, 2012 Even though the exhaust is new I would drop the front pipe and see if it revs. I have seen mufflers with internal blockage cause that problem.RolandYup, Rare, but sometimes they have Mouseitis....and right out of the box too. Dandy Dave! Link to comment Share on other sites More sharing options...
Guest Foggy norm Posted April 9, 2012 Share Posted April 9, 2012 Hey Cowboy...how's it going, got an up-date????? Link to comment Share on other sites More sharing options...
aussiecowboy Posted April 9, 2012 Author Share Posted April 9, 2012 Not yet, I'm still stuck on an oil rig for 10 more days. I have ordered a set of valve springs though, just in case. Link to comment Share on other sites More sharing options...
DavidAU Posted April 9, 2012 Share Posted April 9, 2012 Seems strange that it was going OK but now it is not. I dont think all the valves springs would suddenly all go soft together and cause your problem.You seem to have covered the fuel side of the investigation so apart from the blocked muffler (loose baffle?) I think you hit it when you said it was almost as if it had a governer. What about a couple of spark plug leads hooked up wrong. The engine would probably run OK but not rev or it could be shorting out in the HT wires or a crack in the distributer cap.Give it a run in a dark shed and see what lights up in the HT department. Link to comment Share on other sites More sharing options...
aussiecowboy Posted April 9, 2012 Author Share Posted April 9, 2012 Good suggestions, the cap is new as are the leads and plugs, doesn't mean they are good though. It went bad over the course of several days, it covered about a thousand miles in those days though.I see you have a Metallurgique, a friend of mine went to look at a 27 Buick in an old blokes shed last week and discovered one under a tarp. Very rare car. Link to comment Share on other sites More sharing options...
DavidAU Posted April 9, 2012 Share Posted April 9, 2012 "I see you have a Metallurgique, a friend of mine went to look at a 27 Buick in an old blokes shed last week and discovered one under a tarp. Very rare car. "Thats interesting. They are pretty thin on the ground Link to comment Share on other sites More sharing options...
Stude17 Posted April 9, 2012 Share Posted April 9, 2012 Aussiecowboy have you checked the valve gaps? If the gaps have closed this could be the cause of your problem. Link to comment Share on other sites More sharing options...
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