Jump to content

Hesitation while accelerating under load - self healing


Recommended Posts

Folks, I’m experiencing the type of problem on my TR4a (HS6 carbs), that we all love - one that goes away on its own. 

When starting out on a drive, the car will hesitate when I try and accelerate. Like I’m driving through molasses. No backfires, just bogging down. If I take my foot out of it, the car runs like nothing is wrong. This behavior lasts for about 5 minutes, slowly diminishing as I drive, until it runs and accelerates perfectly. 

Before I start out, the car fires up on the first try with about an inch of choke, and I let it idle until the temp comes up. Then choke off (it idles at 900rpm) and off I go. While sitting still, the car revs fine. It’s only when I give it throttle under load, that the hesitation occurs. I’ve tried adding choke again, but it doesn’t help the hesitation.
 

It’s very disconcerting trying to pull away from a stop light or in stop-and-go traffic, as the car just doesn’t want to respond. So I have to give small inputs with the accelerator. Then after driving for 5 minutes or so, all’s well.

This is the first time driving the car this year, as I had been rebuilding the rear suspension, but I also have installed a new Pertronix ignition module, a new Pertronix coil, new plugs, cap/rotor and replaced the electric fuel pump. I have checked the carb piston oil, and the pistons move freely. 

My initial thought was a vacuum leak, that goes away as the engine gets hot. However wouldn’t a vacuum leak also cause issues sitting at idle, or revving at idle?


Any ideas of what could cause this “self healing” behavior? 

Thanks 

Jim

Link to post
Share on other sites

 

Mine does that if the petrol is old, so perhaps something to do with poor vapourisation until the carbs and manifold get properly warm.  As you say, it goes away as the engine warms up.

 

Link to post
Share on other sites

Bad seal on inlet manifold to cyl head causing air leak when cold which seals itself up as inlet manifold expands. I had exact same on 4A a few years ago, it was the gaskets, got new ones and had to retorque the nuts 3 time over 2 weeks until it finally sealed

Link to post
Share on other sites

Thanks for the responses!

Rob - interesting thought. Though I add gasoline “conditioner” in the fall, it is the fuel from last season.

Ctc - adding choke, or keeping the choke engaged, doesn’t have an effect.

Roger - I checked the dash pot oil after I did the plugs/cap

Chris - I’ll try spraying some starter fluid around the intake manifold flanges and see if I get an rpm change. I’ll check the torque too.


Cheers

Jim

Link to post
Share on other sites

Hi Jim

a couple of years ago (2018 or so) I had serious trouble with my NGK spark plugs.

From new they would last 500-1000miles. I tried several sets but they all failed at about the same time.

I changed to Champion L87 and they last a good 5000 miles.

 

Roger 

Link to post
Share on other sites
50 minutes ago, Tr4aJim said:

Thanks for the responses!

Rob - interesting thought. Though I add gasoline “conditioner” in the fall, it is the fuel from last season.

 


Cheers

Jim

Try a tank of your best octane USA fuel you should notice the difference. 
 

6 month old fuel will go off. Even with a conditioner. 

Link to post
Share on other sites
5 hours ago, Tr4aJim said:

Folks, I’m experiencing the type of problem on my TR4a (HS6 carbs), that we all love - one that goes away on its own. 

When starting out on a drive, the car will hesitate when I try and accelerate. Like I’m driving through molasses. No backfires, just bogging down. If I take my foot out of it, the car runs like nothing is wrong. This behavior lasts for about 5 minutes, slowly diminishing as I drive, until it runs and accelerates perfectly. 

Before I start out, the car fires up on the first try with about an inch of choke, and I let it idle until the temp comes up. Then choke off (it idles at 900rpm) and off I go. While sitting still, the car revs fine. It’s only when I give it throttle under load, that the hesitation occurs. I’ve tried adding choke again, but it doesn’t help the hesitation.
 

It’s very disconcerting trying to pull away from a stop light or in stop-and-go traffic, as the car just doesn’t want to respond. So I have to give small inputs with the accelerator. Then after driving for 5 minutes or so, all’s well.

This is the first time driving the car this year, as I had been rebuilding the rear suspension, but I also have installed a new Pertronix ignition module, a new Pertronix coil, new plugs, cap/rotor and replaced the electric fuel pump. I have checked the carb piston oil, and the pistons move freely. 

My initial thought was a vacuum leak, that goes away as the engine gets hot. However wouldn’t a vacuum leak also cause issues sitting at idle, or revving at idle?


Any ideas of what could cause this “self healing” behavior? 

Thanks 

Jim

My 4 cyl TR does exactly that.  Once thoroughly warmed it is crisp and responsive.

If I richen both carbs by one flat the problem goes away but once warmed through it 'hunts'  Meaning it is too rich.

Winter time I use a thicker oil in the dash pots to keep it rich on pick up. I use 75 weight gear oil.  For summer running I use proper 20 weight dash pot oil

Cheers

Peter W

Link to post
Share on other sites
5 hours ago, RobH said:

 

Mine does that if the petrol is old, so perhaps something to do with poor vapourisation until the carbs and manifold get properly warm.  As you say, it goes away as the engine warms up.

 

Yes I've definitely read that the issue with stale fuel is cold starting/running in particular. The reduction in the volatiles makes the remaining fuel denser than normal which fools the carbs into metering too rich. Which also triangulates with what Peter just said (I think).

Nigel

Link to post
Share on other sites

Ok, I guess I’m buying some gas (uh, petrol)!
 

Roger - I think we’re on the same wavelength. I ditched my NGKs for........wait for it........ Champion L87s!

cheers 

Jim

Link to post
Share on other sites
  • 3 weeks later...

Folks,

Here’s an update to this issue. I drained the tank and put in fresh fuel, but the hesitation issue persisted. Pondering what else could cause this, I decided to check the mechanical advance mechanism in the distributor. My car has a Pertronix distributor, that was installed by the PO. I removed the top internal plate and found that one of the weight mechanisms was broken (with lots of metal “dust” all over the insides). So I guess it’s time for a new distributor.:mellow: I’ll start a new topic on that.

Jim

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.