Sam Buchanan Posted April 5, 2021 Report Posted April 5, 2021 The flattie in my '48 P15 runs strong and smooth. Except.....sometimes at ~50mph on level road under light throttle I can feel (not hear) a slight miss or hesitation. This only happens when the engine is under light load. When I push down the go pedal the engine runs smooth and strong or the hesitation is so slight I can't detect it. Before I start poking around I'm wondering if anyone has seen something similar and has ideas. I've wondered if the carb throttle shaft may be worn enough to allow air to be sucked in under high vacuum. The manifold bolts are tight and I haven't seen evidence of any vacuum leaks. Just trying to ponder this before I mess up by trying to fix something that ain't broke.... ?
chrysler1941 Posted April 5, 2021 Report Posted April 5, 2021 Check your ignition timing. Advance it 5 degrees more and see if it helps. Modern fuel is crap. 1
Sam Buchanan Posted April 5, 2021 Author Report Posted April 5, 2021 Thank you for the suggestion. I suspect our fuel is higher octane than what they had in the 40's, but the timing was a little retarded. I advanced it and initial test drive is promising. 1
Sniper Posted April 6, 2021 Report Posted April 6, 2021 Octane isn't the issue, it's the stuff they are putting in the gas. Ethanol, if you have it in your gas, tends to lean out the mixture. Modern EFI can adjust for that on the fly, carbs cannot. If this is the case you can see symptoms similar to what you are experiencing. Of course any vacuum leak will lean it out too so you may very well be on the right path. 1
greg g Posted April 6, 2021 Report Posted April 6, 2021 It's the flame front. It's slower than old gas. Old stuff went bang! New stuff goes whoomph. So once it's lit off it gives a more sustained push against the piston. As such it does need to be lit off a bit sooner. I did my timing with the vacuum gauge and when checked with the timing light it sits at 5 btdc at idle. I also have a bit of stumble that occures when I lift off the throttle anticipating a down grade or slowing down for a curve. After several attempts to eliminate it, I have chalked it up to slightly unbalanced dual carbs. But it runs so well at all other conditions, I have decided to leave it alone, chalking it up as part of her character flaws. And I would probably make it worse instead of better.
chrysler1941 Posted April 6, 2021 Report Posted April 6, 2021 2 minutes ago, greg g said: It's the flame front. It's slower than old gas. Old stuff went bang! New stuff goes whoomph. So once it's lit off it gives a more sustained push against the piston. As such it does need to be lit off a bit sooner. I did my timing with the vacuum gauge and when checked with the timing light it sits at 5 btdc at idle. I also have a bit of stumble that occures when I lift off the throttle anticipating a down grade or slowing down for a curve. After several attempts to eliminate it, I have chalked it up to slightly unbalanced dual carbs. But it runs so well at all other conditions, I have decided to leave it alone, chalking it up as part of her character flaws. And I would probably make it worse instead of better. Ever hear about Gunson Colortune See Through Spark Plug ? Bought a kit, gonna give it a try this season. One can tune by color of flame. https://www.gunsonshop.com/contents/en-us/p209_G4074-Gunson-Colortune-See-Through-Spark-Plug-Kit.html 2
greg g Posted April 6, 2021 Report Posted April 6, 2021 Interesting!! Keep us informed. You gonna rent those out when you get your sorted???
Sam Buchanan Posted April 6, 2021 Author Report Posted April 6, 2021 1 hour ago, chrysler1941 said: Ever hear about Gunson Colortune See Through Spark Plug ? Bought a kit, gonna give it a try this season. One can tune by color of flame. https://www.gunsonshop.com/contents/en-us/p209_G4074-Gunson-Colortune-See-Through-Spark-Plug-Kit.html I've seen the ColorTunes mentioned in the Triumph TR6 community of which I'm now a member (1974) but I've never felt the need to try them. Yep, the very slight glitch I've noticed is probably too small to continue chasing, I like the idea of chalking it up to "character" since the car runs so well otherwise. ?
Sam Buchanan Posted April 23, 2021 Author Report Posted April 23, 2021 (edited) Earlier I posted: The flattie in my '48 P15 runs strong and smooth. Except.....sometimes at ~50mph on level road under light throttle I can feel (not hear) a slight miss or hesitation. This only happens when the engine is under light load. When I push down the go pedal the engine runs smooth and strong or the hesitation is so slight I can't detect it. Before I start poking around I'm wondering if anyone has seen something similar and has ideas. I've wondered if the carb throttle shaft may be worn enough to allow air to be sucked in under high vacuum. The manifold bolts are tight and I haven't seen evidence of any vacuum leaks. Just trying to ponder this before I mess up by trying to fix something that ain't broke. Update: The slight hesitation is still present at 50-55mph under light throttle. It feels like the engine is running a little lean when a nearly closed throttle is creating a lot of vacuum and now I really suspect that is the case. If I pull out some choke the engine smooths out. Now...where is the vacuum leak occurring? It seems to me it could be any of the following: Wear in the carb at the throttle shaft. But it feels tight. A leak somewhere in the wiper system. I can easily plug that line as a check. A leak in the vacuum advance unit. That one is a little harder to diagnose, guess I could plug that line and run it just on centrifugal advance. However that might introduce other issues that complicates diagnosing. A manifold leak. Could be, but everything is tight and it got a new gasket a year ago. Any thoughts? Edited April 23, 2021 by Sam Buchanan
Dartgame Posted April 24, 2021 Report Posted April 24, 2021 Vacuum advance is easy to check. Backyard mechanic method. Remove the vac adv line from the carb find a piece of small rubber hose that fits tightly on the loose end of the vac line. Then using your mouth pull some vacuum (suck) on the rubber hose and let your tongue seal it. hold it and see if it maintains the vacuum you applied for 10-20 seconds. If it does then you are good to go. More sophisticated way of checking - get a hand operated vacuum pump (mighty vac) and substitute it for the mouth portion of the procedure above, these have a vacuum gauge which will hold the reading if the adv unit is good. To find other leaks get some carb cleaner spray. While running the engine, squirt a little of the spray around the suspect items. Be careful of fire hazard, dont use too much spray. if the engine changes speed you have found a leak. 1
kencombs Posted April 24, 2021 Report Posted April 24, 2021 If it is due to a lean condition, it may not necessarily be due to a leak. Just not enough fuel being metered for the airflow at that specific speed. You may be able to help by raising the float level a slight amount. Faster, easier and cheaper than a jet change. Do you by any chance have an added PCV system? If so, that valve can cause issues if not well matched to the use. If none of the above, I'd suspect the fuel metering through the power enrichment circuit ( I think Mopar calls it 'step up') may be in play. Could be that at that specific speed the vacuum level is marginally triggering that valve causing a stumble. If the stumble happens on a road grade change while maintaining a constant speed that would be and indication of that. I've never seen a comparison of 40-50s Mopar carbs settings for that, but there may be different springs available from various cars that used the same basic carb.
Sam Buchanan Posted April 24, 2021 Author Report Posted April 24, 2021 4 hours ago, kencombs said: If it is due to a lean condition, it may not necessarily be due to a leak. Just not enough fuel being metered for the airflow at that specific speed. You may be able to help by raising the float level a slight amount. Faster, easier and cheaper than a jet change. Do you by any chance have an added PCV system? If so, that valve can cause issues if not well matched to the use. If none of the above, I'd suspect the fuel metering through the power enrichment circuit ( I think Mopar calls it 'step up') may be in play. Could be that at that specific speed the vacuum level is marginally triggering that valve causing a stumble. If the stumble happens on a road grade change while maintaining a constant speed that would be and indication of that. I've never seen a comparison of 40-50s Mopar carbs settings for that, but there may be different springs available from various cars that used the same basic carb. The float is set just a bit below book in an effort to reduce percolation, but the engine runs strong at full throttle with no signs of fuel starvation. No PVC system, just the standard chassis anti-corrosion system. Yep, the hesitation only occurs on the highway in a very narrow range of light throttle and low engine load. It can't be heard, just felt. Add some throttle and the engine pulls strongly. Drive into a headwind that requires more throttle and the hesitation doesn't occur. Maybe I just need to consider it a "feature", not a problem. ? Thanks to all for the suggestions.
greg g Posted April 25, 2021 Report Posted April 25, 2021 Is the step up valve in the carb moving freely?
Sam Buchanan Posted April 25, 2021 Author Report Posted April 25, 2021 10 minutes ago, greg g said: Is the step up valve in the carb moving freely? Don’t know, haven’t been in the carb since I returned the car to service two years ago. I’ll need to do some homework.
martybose Posted April 25, 2021 Report Posted April 25, 2021 I'm surprised that no one has mentioned that this could simply be a non-functional accelerator pump. Your description is exactly what my car felt like with the original carb until I put in a new accelerator pump piston. Marty
Los_Control Posted April 25, 2021 Report Posted April 25, 2021 In a perfect world, you should see a generous stream looking into the throat and working the throttle. Should be easy enough to see if it is working. The main purpose of the accelerator pump .... when you are driving and step on the gas, you open the throttle plate and instantly get more air volume. The accelerator pump just gives you a xtra squirt of fuel to compensate for the sudden burst of air. Then the main jets take over. A very over simplified explanation. If the accelerator pump is not working properly, when you step on the gas you get more air and a lean fuel mixture until your main jet catches up. Otherwise explained as a hesitation.
Sniper Posted April 25, 2021 Report Posted April 25, 2021 I read the OP's original post as a steady state but light throttle not as accelerating and I believe he specifically stated if he accelerated it went away. Which would rule out the accel pump, in my thinking.
Los_Control Posted April 25, 2021 Report Posted April 25, 2021 To be honest, I read the thread title a few days ago when appeared. Thought to myself, there are many here that are 10 times smarter then me to help on this problem. I never read the actual thread. When I saw @martybose post I just thought would add a little to it. Why you need a working accel pump, & takes 2 seconds to check & see if yours is working. Growing up with carburetors we are just use to them .... may have been @keithb7recent video that inspired my response 1
Sniper Posted April 25, 2021 Report Posted April 25, 2021 Heck, I'm not saying you're wrong, just offering up where my thoughts are.
Sam Buchanan Posted April 25, 2021 Author Report Posted April 25, 2021 Yep, the hesitation is only under light, steady throttle (as I have stated several times ). The engine accelerates just fine, accelerator pump is working fine.
greg g Posted April 25, 2021 Report Posted April 25, 2021 Carter or Stromberg? What do the plugs look like? Could it be initial over rich till airflow addresses with increased air to balance mix.
Dan Hiebert Posted April 25, 2021 Report Posted April 25, 2021 Our D24 does the same thing. After many unsuccessful attempts to "fix" it, I too, decided long ago to chalk it up to "character" and leave well enough alone. Runs great otherwise.
Sam Buchanan Posted April 25, 2021 Author Report Posted April 25, 2021 1 minute ago, greg g said: Carter or Stromberg? What do the plugs look like? Could it be initial over rich till airflow addresses with increased air to balance mix. Using some choke to smooth out the hesitation leads me to think it is going lean. The condition is so slight and happens so infrequently I doubt reading the plugs would show much, especially if lean. Carter.
Sam Buchanan Posted April 25, 2021 Author Report Posted April 25, 2021 4 minutes ago, Dan Hiebert said: Our D24 does the same thing. After many unsuccessful attempts to "fix" it, I too, decided long ago to chalk it up to "character" and leave well enough alone. Runs great otherwise. Interesting. I'm leaning toward the same conclusion. Anything 73 years old has earned the right to demonstrate some character. ? 1
chrysler1941 Posted April 25, 2021 Report Posted April 25, 2021 On 4/23/2021 at 2:52 AM, Sam Buchanan said: The slight hesitation is still present at 50-55mph under light throttle. My carb is no different than yours and have no hesitation, so don't live with it.? How about fuel delivery? At these speeds your engine is thirsty. Could be carb float height, clogged passages, jets. Fuel line, Tank. Pump, filter, swollen rubber hose. If carb is ok, try adding an electric fuel pump as a test. Not any but one with max pressure 4.5 PSI like Airtex E8902
Recommended Posts