• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Engine misses under load

Share this page

7

70herald

Luke Skywalker
Country flag
Offline
Hi
Well after 2 years of having the Herald rather disassembled. I finally got the first drive in today. First over to the mechanic to get the front end aligned- it handles real nicely now. Tight and precise (all the bushings are new) Still need to figure out why the brakes are not quite up to par.

However the engine is missing badly under load. At idle is seems fine. Even on level ground, were there is minimal load, it didn't seem to miss. However going up hills at any sort of speed is virtually impossible. It misses and surges.
the only thing which was not rebuilt was the distributor. the shaft doesn't seem to wiggle. Should this be the first suspect? Timing? How about fuel mixture?
 
My first thought would be inadequate fuel delivery, a restriction of some sort that passes enough fuel for idle or cruise, but not enough for full power.

Weak ignition can cause the same symptoms, but I'd check fuel first. One easy way to check is to tee in a suitable pressure gauge (most vacuum test gauges will read fuel pressure as well) right at the carb and bring it out where you can see it while driving. If it still reads over 1 psi when the problem occurs, then you know it's not fuel delivery (at least up to the point where the gauge is connected).
 
Everything in the fuel system is new, from the rebuilt pump, nice clean filter, hoses, and the "big" carb rebuild kit with all the parts (shafts, valve the brass disk that wears out..)

Could the fuel filter (one of those folded paper in a plastic container) be causing enough of a restriction that that this could happen even though it is clean?
 
Did you check all the new fuel lines to be sure none are kinked?
 
Yisrael:

I had the very same problem three weeks ago.

It was finally diagnosed by That Canada Guy as a failing
Petronix ignition and fuel restriction thru the new high
tech, twin, carb filters.

Installed points and removed the carb fuel filters.
The engine now screams and will catch rubber going into
second gear... if I push it too hard.

My vote goes to fuel restriction.

d
 
Check your dwell. I'll bet it's off. Remember to re-time the engine after.
 
Dwell is dead on. 59 degrees spec is 60±2. I don't think there are any kinks in the fuel line but I will double check and take out the filter just to make sure.
 
I'd make sure you're getting spark to all the cylinders just to be sure, then check timing, and rev it up and make sure your advance mechanism is working. If that all looks good, check your carb piston(s?), make sure it's not stuck and it's moving smoothly, and you've got oil in the damper tube. Then I'd check your fuel mixture.
 
One other thing to [re]check, if you haven't already (and if, indeed, this is applicable) is the diaphragm in the Stromberg I assume your 13/60 has. Tears and perforations in same can cause what you're seeing. And I still wouldn't rule out some sort of other restriction somewhere in the fuel system.
 
The carb diaphragm is new, as is virtually everything else. While Dale was building the worlds most expensive TR6, I was building the most expensive Herald.
Advance is kind of screwy on this carb. The engine was originally a low compression (7:1) with the oversize pistons, and the head slightly shaved I am at about 8:1. In any case, the dizzy has a maximum mechanical advance of 7 degrees, and the vac advance should give 12 degrees. It is advancing to about these angles but the mechanical hits its max by about 1500 rpm instead of 2000. In any case, as some point fairly soon, I will be in the states and will bring the dizzy along so that Jeff at Advance Distributors can work out a more reasonable curve for it. In any case, the curve should only cause it to be somewhat inefficient.

My first step will be to get rid of the filter and see what happens. It kind of makes sense, if the filter was designed for a more modern vehicle, it could very well be to restrictive, modern electric pumps have far more pressure available!
 
"Even on level ground, were there is minimal load, it didn't seem to miss. However going up hills at any sort of speed is virtually impossible. It misses and surges. "

This seems a major clue. I know zilch about Heralds, but could there be something loose in the fuel tank and/or float bowl that might be blocking fuel flow, but only at an angle? If you rev the engine while parked, same problem?

Tom
 
Tom,

My engine would rev perfectly in the garage and easily
held 3000+ rpms for two minutes or more..

Out in the street, under load, everything fell to pieces
on a slight incline on a side street.

Failing Petronix and restricted gas flow thru the new carb
filters were the culprits. In addition, we were only three
blocks from DPO Pedro's house.

What can it hurt to pull the fuel filter off and see what
happens out in the street?

David and I figured the surging was the result of the engine
sucking the carb bowls dry--sputter-sputter-sputter, until
the fuel pump slowly filled the bowls again-surge in power
until bowls drained again-etc.

d
 
Well, there you have it!! Spelled out simply enough.

Your problem is either:
1. fuel related
2. electrical related

or

3. combination of 1 and 2

Whew, that was an easy solution! But I highly recommend
you fly That Canada Guy over to Israel for a consultation,
just to be sure.

d /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif
 
My TR3 did the same thing and what was happening was the plugs were fouling. Replaced the recently rebuilt carb with an older one and reset the fuel mix. Problem solved.

From my 30 years experience with engines (race engines) rebuilt doesn't mean good. If the solution isn't apparent in the next few changes find someone with another set of carbs and change them out.
 
Yisrael - what's the latest? How did the filter-less test go?

Tom
 
We can but hope DPO Pedro was occupied elsewhere
and Yisrael is but a cloud of churning dust- far
down the road.

/bcforum/images/%%GRAEMLIN_URL%%/driving.gif
 
NutmegCT said:
Yisrael - what's the latest? How did the filter-less test go?

Tom
Click to expand...
I spent the day pulling the head back off again. Not a happy camper.
I brought it over to my friendly local shop so that we could put it on the analyzer, and it was missing at idle. We opened up the oil filler cap and it was coated with mayonnaise. /bcforum/images/%%GRAEMLIN_URL%%/mad.gif
It was also quite clear that water was leaking into the cylinders and THAT is problem #1. Anyway, I pulled off the head, and since I didn't have a new head gasket, I coated it with gasket shellac (temporary measure only!) and put it all back together. It works somewhat better (leakage is significantly reduced but I need to get a new head gasket since there clearly is still a blowby issue also, and while I am at it, I guess the head goes back to the machine shop for a bit more machining.
 
70herald said:
[
Click to expand...
We opened up the oil filler cap and it was coated with mayonnaise.
[/quote]

OUCH!! Seriously Pedro-ized. No wonder my starter install
went so smoothly. That's a real bummer Yisrael.

d
 
Back
Top