Spitfire Speed and Power 75 spit

[TR3driver]

timing or fuel delivery.

double check your static timing,

about 10 degrees advanced at idle.

So, what does it do as you rev the engine up?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]

I gotta fried chicken tv dinner that you are going way lean at the top end either because the fuel pump isn't, the fuel filter is clogged or the main jets are too small.

Carb origins and jet sizes are unknown...PO install[/QUOTE]One way to check dynamic mixture is to do "plug cuts" : Run the engine under the condition of interest for a bit, and then immediately drop the clutch, turn off the ignition and coast to the side of the road. Obviously it's best to do this somewhere away from traffic. Then you can remove the spark plugs and read them for mixture. Not as accurate as a good wide band O2 meter, but cheap and effective.

If you suspect fuel delivery problems (bad pump, crushed line, clogged filter, etc.), get a gauge that can read fuel pressure (eg https://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html ) and tee it into the line at the carb using a line long enough that you can lead it out from under the hood and prop it under a wiper. Obviously this isn't a permanent setup, but now you can take a test drive and see what is actually happening to fuel pressure.

 

[chappy444]

Bill,

Stock spit cat is located right at the exhaust manifold.

if you go to the Victoria British catalog it gives a nice pic of what the pipe looks like.

Monza exhaust has a resonator mid pipe and resonator/mufflers right before the tips.

my car has the manifold like the one pictured on the lower left of page 61 of the spitfire VB cataloge.

it does not have the pipe with the bulge at the top like the one pictured on the top left of page 136 of the VB cataloge....

i have a straight pipe with a flange on it connecting the mid car resonator with the end of the exhaust manifold...unless the cat is part of the actual manifold i do not think i have one installed on this car.

 

[chappy444]

So what does it do as you rev the engine up?

i have the Pertronix Ignitor in place of the points and condenser in a Delco D204 distributor. the timing advances with revs (total advance i have seen is about 12 degrees form 10 advanced at idle to just off the scale under hard rev) i have tried running with the vac retard connected and disconnected and i could not notice a real difference.

One way to check dynamic mixture is to do "plug cuts" : Run the engine under the condition of interest for a bit, and then immediately drop the clutch, turn off the ignition and coast to the side of the road. Obviously it's best to do this somewhere away from traffic. Then you can remove the spark plugs and read them for mixture. Not as accurate as a good wide band O2 meter, but cheap and effective.

If you suspect fuel delivery problems (bad pump, crushed line, clogged filter, etc.), get a gauge that can read fuel pressure (eg https://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html ) and tee it into the line at the carb using a line long enough that you can lead it out from under the hood and prop it under a wiper. Obviously this isn't a permanent setup, but now you can take a test drive and see what is actually happening to fuel pressure.

i have a a filter at the tank, one after the pump...then i have pressure gauge. right before the carb i am getting about 3.5 PSI... i back blew the tank pickup tube to make sure i wasn't getting a clog at the internal filter. fuel delivery to carb seems good. for vac am getting about 5in Hg at idle and about 15 in Hg under rev...this is taken at the port on the bottom plate of the carb.

 

[Don_R]

Bill,

Sounds like no cat for you.

As Randall suggested, what does your timing do as you rev up?

Get your RPM as low as possible (700 or so) and watch the timing mark as you bring up the RPM to about 2500 or so. you are lookin for the timing to advance as the rpm comes up.

This should confirm that the mechanical advance is working and not stuck. i think you should see about 15 degrees of advance.


are you running Points or electronic ignition?

Make sure the points are set correctly. I use a dwell meter for mine. 38-42 degrees dwell on the points?

Valve lash affects the timing. Have you check the cold lash on the valves?

 

[chappy444]

Bill,

Sounds like no cat for you.

As Randall suggested, what does your timing do as you rev up?

Get your RPM as low as possible (700 or so) and watch the timing mark as you bring up the RPM to about 2500 or so. you are lookin for the timing to advance as the rpm comes up.

This should confirm that the mechanical advance is working and not stuck. i think you should see about 15 degrees of advance.


are you running Points or electronic ignition?

Make sure the points are set correctly. I use a dwell meter for mine. 38-42 degrees dwell on the points?

Valve lash affects the timing. Have you check the cold lash on the valves?

i have the Pertronix Ignitor in place of the points and condenser in a Delco D204 distributor. the timing advances with revs (total advance i have seen is about 12 degrees form 10 advanced at idle to just off the scale under hard rev) i have tried running with the vac retard connected and disconnected and i could not notice a real difference.

as for the valve lash...that was one of the first things i did when i brought it home.

 

[71MKIV]

ok, now that the simple things are sorted out,

it's possible depending on the dpo, that the timing chain was out and is put back in one tooth off. Which would make it breath terrible.

I'd have to take my carb apart to tell you what jets and tubes I am running.

I took the center resonator off and replaced it with a aftermarket cherry bomb type. mellowed out the sound of the monza without taking away the growl. The monza for all intents and purposes is an open pipe.

 

[tomgt6]

I would try a rebuild of the carbs. Something is gummed up is my guess. I don't know much about the webers but maybe a high speed jet or something like that is clogged. It seems like you have power at low rpms but when you need more it goes away.

 

[TR3driver]

right before the carb i am getting about 3.5 PSI...

Is that under full power?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] for vac am getting about 5in Hg at idle and about 15 in Hg under rev...this is taken at the port on the bottom plate of the carb.
[/QUOTE]
Something seriously wrong there, you should be getting over 15" at idle. I'm guessing that port is actually a vacuum advance port; but it could also indicate a major vacuum leak or bad valve timing.

Another possibility might be that the front pulley has slipped (meaning the ignition timing is actually way late in spite of what the mark shows). Might be worth double-checking that the mark is accurate, using a piston stop.

But I'd look for a manifold vacuum port first, like maybe a PCV valve or brake booster fitting. On my TR3A there was no manifold port originally, so I just drilled & tapped the manifold for a port (which of course has to be capped off when not in use).

 

[TR3driver]

Also, while most folks have no trouble with them, the Pertronix made my TR3 run noticeably worse. If nothing else seems to work, I'd try going back to points.

 

[chappy444]

Is that under full power?

that is sitting in the garage in neutral...no load...at idle...when i rev it up it will pump up to just under 5psi...its a cheap in-line Mr.Gasket gauge...

Something seriously wrong there, you should be getting over 15" at idle. I'm guessing that port is actually a vacuum advance port; but it could also indicate a major vacuum leak or bad valve timing.

Another possibility might be that the front pulley has slipped (meaning the ignition timing is actually way late in spite of what the mark shows). Might be worth double-checking that the mark is accurate, using a piston stop.

But I'd look for a manifold vacuum port first, like maybe a PCV valve or brake booster fitting. On my TR3A there was no manifold port originally, so I just drilled & tapped the manifold for a port (which of course has to be capped off when not in use).

this is only Vac spot on the car that i can find...i have the Weber DGV conversion on a pierce manifold...this vac port is on the carb...not the manifold. and it runs to that stupid vac retard can on the distributor. it is my understanding that the retard can is for fuel economy/emmissions.

as far as timing goes...there could be a problem...to make sure i have my process in order to check this...
i need to...
remove the valve cover and spark plugs
move #1 piston to TDC just after #1 intake vavle closes and check that the rotor button is just hitting #1 post on cap (about 10 o'clock on my car)

 

[chappy444]

I would try a rebuild of the carbs. Something is gummed up is my guess. I don't know much about the webers but maybe a high speed jet or something like that is clogged. It seems like you have power at low rpms but when you need more it goes away.

when i bought this car a month ago it would idle but not drive...it would fall on its face as soon as you put a load on it...i could get it to idle around the street infront of my house but could barely getit back up the driveway adn into the garage....

5 cans of carb cleaner...a few total dissasemblies, one rebuild kit (including pump valve diagphragm and main pump diaphragm) many float adjustments and a few days of idle mix/idle/timing adjustments later and i could drive the thing up the road...which lead me to my current problem....

on the weber there is a primary fuel jet, secondary fuel jet, primary air jet at the top of the primary emulsion tube, and a sedcondary air jet at the top of the secondary emulsion tube. i am positive that kept these jets in the same position as they were when i took them out...the PO "said" that he cleaned the carb to try to fix the problem it had when i bought it...but it did not look to me like it had ever been cleaned...is it possible that thejets could have been reversed at some point?...would this cause my symptoms anyone know which side the larger jet should go on??? larger fuel jet on primary or secondary? same for the air jets????

 

[TR3driver]

that is sitting in the garage in neutral...no load...at idle

My point is that you need to be sure of adequate fuel when the fuel is actually flowing at maximum rate, IOW while the engine is under full load.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]and it runs to that stupid vac retard can on the distributor.[/QUOTE]A vacuum retard port would be setup so it only supplies vacuum at idle; meaning the retard is only active at idle. From your measurements above, this is NOT a retard port and should NOT be connected to the retard module.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] it is my understanding that the retard can is for fuel economy/emmissions.
[/QUOTE]What the retard module does is yank the timing back by about 15 degrees from where it would be otherwise. It was intended as an emissions control device, to reduce unburned hydrocarbons at idle. It actually burns more fuel (but in theory more completely), so it's only impact on economy is negative.

However, if your DPO had connected it to the advance port, then the retard would become active while driving, rather than at idle. 15 degrees of retard would kill both performance and fuel economy.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]as far as timing goes...there could be a problem...to make sure i have my process in order to check this...
i need to...
remove the valve cover and spark plugs
move #1 piston to TDC just after #1 intake vavle closes and check that the rotor button is just hitting #1 post on cap (about 10 o'clock on my car) [/QUOTE]
No, you have no need for that. That is the check for firing the cylinder that is near TDC on compression (rather than the one that is near TDC on exhaust). Since it runs at all, it's firing the right cylinder.

 

[Bob Claffie]

I'm with TR3 guy on the vac timing. You should switch to a dizzy with a vac ADVANCE, not retard. Perhaps just change the vac canister on your existing dizzy. If you do that be sure to use manifold vac and not ported vac. Bob

 

[Bob Claffie]

I had a "stock" '76 a number of years ago and while it wasn't very fast it wasn't disgustingly slow either. If I still had it I would get a British manifold with SU's and dump the original Stromberg (feel much the same way about aftermarket Webers, yuk.) Bob

 

[chappy444]

thank you all for the helpfull responses...

I am now convinced it is a carb/fuel delivery issue....(ok...maybe ingnition)

at 4000+ RPM it starts to "break up" no longer a steady exahust note...almost like running up against a govoner or something...the power dosen't fall off really but it won't make anymore power, and it won't rev higher.

starved for feul? jets too small? fuel pressure not enough?

the joy continues...but at least i feel like i am making progress..

 

[poolboy]

Add "linkage adjustment" to that list.

 

[chappy444]

Add "linkage adjustment" to that list.

yeah. did some of that last night...i think i have it close to the optimal point...where i get the most travel...open the secondary fully, not getting off the idle speed screw, only using the idle circuit at idle and not getting into the primary...

i am thinking that it is either starved for fuel or getting to much at WOT above 4000rpm....
wouldn't too much fuel cause it to fall on its face and bogg down?

it seems like i have the opposite problem...it won't rev any higher but it dosen't loose any ground on the power...

i am totaly not a carb guy... i have been on a steep learning curve with carbs since i got this car.

Bill

 

[tomgt6]

Don't feel bad. I have been trying to get my SUs tuned for the Spitfire for the last week and have alot of the same issues. I am learning to hate carbs.

 

[TR3driver]

at 4000+ RPM it starts to "break up" no longer a steady exahust note...almost like running up against a govoner or something...the power dosen't fall off really but it won't make anymore power, and it won't rev higher.

starved for feul? jets too small? fuel pressure not enough?

Not likely to be a jetting problem, IMO. Unless the carb is too small, it should produce the same mixture at 4000 rpm that it does at 3000 rpm. So jetting errors tend to be roughly the same over the entire power band, or get gradually worse from one side to the other. They rarely produce that kind of "wall" effect where the engine suddenly stops pulling at a fairly specific rpm.

Edit: The exception would be carbs with a vacuum operated secondary; but AFAIK all DGVs have a mechanical linkage to the secondary.

If you run up at full throttle, and then back off to perhaps half throttle, does it seem to "catch it's breath" and start running smoother? That is a classic symptom of inadequate fuel delivery (which might be as simple as some water or bad fuel getting into one of those filters).

Given that it kind of sounds like this engine has had problems for a long time, I might also look at valve springs. They do get tired and weak with time; and weak springs will let the valves float at higher rpm (leading to the wall effect). They can also lead to burnt valves if not taken care of.