Lack of pull above 4500 revs

[DazRR]

Hi all,My 57 Road Rocket lacks top end power, I've looked into all potential problem areas (I think). I rebuilt the engine approx 2000 miles ago, resleeved with 9:1 pistons, reground crank with new big ends and new timing side bush, it has a 356 cam and the cylinder head is a 67-1549 from a super rocket. Timing set at 34 degrees on electronic ignition on full manual advance, Carb is a 389 1" 1/8th with 420 main jet. Gearing is standard apart from a 22T engine sprocket (22-43-19-42). Plug chop at all throttle intervals shows a dark tan plug, I am not the lightest pilot at 18 stone but i would have though it should pull at least 80mph, I do have poor rocker to valve geometry on the inlets, almost as if the pushrods are slightly too short however they measure correctly at 8.5 inches, any ideas why it is lacking some get up and go ?

[Greybeard]

Just a couple of quickies:
Have you tried opening both petrol taps?
Have you checked that the filler cap vent is clear?

[DazRR]

Just a couple of quickies:
Have you tried opening both petrol taps?
Have you checked that the filler cap vent is clear?

Hi Greybeard thanks for your reply, yes have tried with both taps open and the filler cap vent is clear.

[DazRR]

Hi DazRR

You don't mention your carb needle position, or cutaway, also check your needle type carefully.

420 is a huge main jet, have you tried say a 360 and your carb would be 1 5/32" on a late SR head.

1 1/8" is fine but could mean a bit of tinkering from std jets, needle positions etc.  357 cam would suit your motor better with 9 to 1 pistons, I don't know if the 356 is a problem necessarily.

P.S. Just trying to think of easy fixes before you recheck valve timing / ignition setting.  Also make sure your throttle cable is taking the slide up sufficiently, also your choke is out of the way but I am sure you know this stuff.

P.S.S.  1960/63  SR/Spitfire 389 1 5/32, main 290 (RGS 310), pilot 25, slide 3, N.Pos. 2,  N.Jet. .106

Thank you for trying to help, I have based my carb jetting on the attached, I am running without an air cleaner. I have run it with a 320 main however the plug chop was too light a tan for my liking

[Triton Thrasher]

Fuel flow should be at least 250cc/minute through the bottom of the carburettor with main jet cover removed.

Generally, you get a suitable main jet size by going too big, so that at full throttle it 8-strokes/soots up the plug, then fit progressively smaller jets until it runs properly.

A sign of slight richness on the main jet is when you close the throttle from fully open to 7/8 open: you feel a dip in power, then a recovery.

If your 22t sprocket makes your bike higher geared than standard and you’re not Twiggy, I think you should expect poor top gear performance, even with perfect fuelling.  How fast can it go in third gear?

[DazRR]

Fuel flow should be at least 250cc/minute through the bottom of the carburettor with main jet cover removed.

Generally, you get a suitable main jet size by going too big, so that at full throttle it 8-strokes/soots up the plug, then fit progressively smaller jets until it runs properly.

A sign of slight richness on the main jet is when you close the throttle from fully open to 7/8 open: you feel a dip in power, then a recovery.

If your 22t sprocket makes your bike higher geared than standard and you’re not Twiggy, I think you should expect poor top gear performance, even with perfect fuelling.  How fast can it go in third gear?

I'm deffo not Twiggy LOL, I will do a check of fuel flow rate, that's something I have overlooked, thanks for your help.

[muskrat]

G'day Daz.
What grade fuel are you using?
The higher the octane the slower it burns so advance timing.
The higher compression the faster the burn so retard the timing.
Try altering it 2 degrees one way then the other.
"Timing set at 34 degrees on electronic ignition on full manual advance," Advance is controlled by the black (sometimes red) box. What breed of ignition do you have?
A 357 cam will give better top end, comes in at about 4500rpm but takes away from low grunt. Check your valve timing.
Cheers

[Brian]

One thing worth checking is valve timing. You need to set up a degree wheel and make sure they open and close when they should.

I have had your symptoms twice, once with a long stroke engine with the wrong idler gear and once with a reground cam.

The engine will start easily, idle perfectly and run faultlessly up to about 55mph and then just wont go any further.

[DazRR]

G'day Daz.
What grade fuel are you using?
The higher the octane the slower it burns so advance timing.
The higher compression the faster the burn so retard the timing.
Try altering it 2 degrees one way then the other.
"Timing set at 34 degrees on electronic ignition on full manual advance," Advance is controlled by the black (sometimes red) box. What breed of ignition do you have?
A 357 cam will give better top end, comes in at about 4500rpm but takes away from low grunt. Check your valve timing.
Cheers

I'm using E5 fuel which is supposedly 97 octane, with the manual advance set to full it will pink on gradients so the happy spot seems to be retarded slightly from this point. It has a thorspark ignition kit so there is no black box,advance is all manual. Thanks for your thoughts.

[DazRR]

One thing worth checking is valve timing. You need to set up a degree wheel and make sure they open and close when they should.

I have had your symptoms twice, once with a long stroke engine with the wrong idler gear and once with a reground cam.

The engine will start easily, idle perfectly and run faultlessly up to about 55mph and then just wont go any further.

Once I've ruled out fuel flow this will be my next line of investigation, the cam is unknown to me, it looked in very unworn condition but that doesnt mean its profiles are correct, thank you for your reply.

[bikerbob]

Putting a bigger engine sprocket can reduce the top end speed in top gear, I am reminded that way back in the early sixties me and my mates were always trying to get more top end speed sometimes by changing the engine sprocket one mate in particular had a Norton 99 he increased the size of the engine sprocket and it did go faster in  the first 3 gears but when he went from maximum speed in  third then into top gear it slowed down by about 5mph.. As regards the camshaft it should have a 356 cam fitted and it is usually stamped on the end of the cam shaft the 357 was a sportier cam but it is possible that maybe a previous owner has fitted a 354 cam.

[DazRR]

Putting a bigger engine sprocket can reduce the top end speed in top gear, I am reminded that way back in the early sixties me and my mates were always trying to get more top end speed sometimes by changing the engine sprocket one mate in particular had a Norton 99 he increased the size of the engine sprocket and it did go faster in  the first 3 gears but when he went from maximum speed in  third then into top gear it slowed down by about 5mph.. As regards the camshaft it should have a 356 cam fitted and it is usually stamped on the end of the cam shaft the 357 was a sportier cam but it is possible that maybe a previous owner has fitted a 354 cam.

I understand that a larger sprocket will impact on top speed however I'm only 1 tooth larger, the cam is deffo a 356 as I've rebuilt the engine,picture of end stamping attached.

[Joolstacho]

Daz, though your camshaft is marked as a 356 (or whatever) it's possible that it's been re-ground, so it's not guaranteed. Measuring lift and using a degree wheel to check valve timings is the guarantee. And tappet adjusters may give you a clue too. Note your comment: "I do have poor rocker to valve geometry on the inlets, almost as if the pushrods are slightly too short". - Quite possibly a cam re-ground issue.

[DazRR]

Daz, though your camshaft is marked as a 356 (or whatever) it's possible that it's been re-ground, so it's not guaranteed. Measuring lift and using a degree wheel to check valve timings is the guarantee. And tappet adjusters may give you a clue too. Note your comment: "I do have poor rocker to valve geometry on the inlets, almost as if the pushrods are slightly too short". - Quite possibly a cam re-ground issue.

Thanks mate, this makes sense, I will do some measuring during the winter and if the valve timing and lift is wrong I'll do another tear down and change the cam.

[KiwiGF]

Was the air vent to carb bowl checked? (In tickler)