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MG MGB Technical - 3,000 rpm Balk

Gentlemen;

'72 18V; Peter Burgess Fast Road Head; HS6 carbs, K&N air filters; BCF needles with yellow springs; Euro spec D45 Dizzy with points @14, dwell @ 48; Lucas sport coil, plugs gapped to 38. The car ran great (70 mph in 3rd)until I added a Piper 270 cam with vernier gear timed in to 108 ATDC. Suddenly the car runs great to 2,800 RPM, balks at 3,000 RPM and will lurch, missing, spitting to 4,000 not will not go beyond 4,000 RPM.

I have good compression in all cylinders; tightened all manifold bolts and exhaust connections; tuned the HS6's with (& without)the gunasion tool to run either rich or lean, swapped both needles (BBW to BCF and back) and springs (stock blue to yellow and back)independently and together; have swapped coil, plug wires, plugs. I have timed the dizzy everywhere from 5 BTDC to 25 BTDC; I have swapped the in-line fuel filter, fuel pump filter and blown out the fuel lines from the tank to the pump and from the carbs back to the pump. No matter the change, the engine still balks at 3,000 RPM.

Do I have a bad cam? Insufficient fuel pump? Bad dial guage? Bad karma?
Bayard DeNoie

Bayard-
If you got your Eurospec distributor from Brit Tek, then it's really an Aldon-modified unit, the 101BY1. With a Piper 270, you really should be using the 101BR2. Those HS6's are little large for the engine that you describe. However, you may have a bad camshaft. What bedding-in procedure did you follow?
Steve S.

Bayard "The car ran great[70 mph in 3rd]until I added a piper 270 cam" thats like saying "I don't understand officer, he was fine till I shot him." check the cam and cam timing. It really sounds like that dead end at 3000rpm is cam related.RIC
R E L Lloyd

If the 45D has the vernier adjustment nut it could be an indication that your distributor is not properly set up. As the R.P.M.’s increase the rotor may be too far advanced which causes the spark to jump to the wrong post in the distributor cap...either too late or cross fire (misfire). This can be corrected by first, degreeing the rotor, then secondly, adjusting the timing by turning the distributor. Fine incremental spark adjustments can be made with the vernier adjustment nut but this should never be used to “dial in” (set) the timing.
The vernier adjustment nut moves the base plate in the distributor which changes the position of timing in the distributor ... when or where the spark jumps from the rotor to the post in the distributor. This is more likely to show up at higher R.P.M.'s than 3000 to 4000 but it's possible. It's the first area I would check out.
Ribert Ritter

Steve S:

I originally ran the Weber DGV and was very disappointed with performance. I wanted to get back to SU's and wanted a 286 cam - So I bought the HS6's once the head came back from the UK. The new head and HS6's worked well with the stock cam. There was quite a bit of controversy over the 286 and I recalled your having been pleased with the 270 so went in that direction. While I have always done my own work, I did not have the time (or the inclination) to do the cam and had a mechanic affiliated with our local MG club put in the cam. When I picked up the car, after the new cam had been added, I had the feeling that it had been installed one tooth off. I went to pull the crank bolt to put in the vernier gears and decided I needed to replace the clutch. With the engine and out, I re-timed the cam using the procedures in Burgess' book. My distributor did come from Brittek. Do you feel the 101BY1 would cause the problem described or just not give peak performance?

REL: I think it is the cam or cam timing as well. No one seems to consider lack of fuel a possibility.

Ribert: Thanks for your input. I re-timed the dizzy at 5 degree intervals forward from 15 degrees to 25 degrees BTDC and then back again to 5 degrees and then forward again to the original 15. I tuned the carbs at every 10 degree change. Since the balk comes at exactly the same point regardless of whether the timimg is set to 5 BTDC or 25 BTDC, I do not suspect small adjustments will have much impact on the 3000 RPM balk.

I suspect it is time to bring the car to a professional who can determine if I have a faulty cam, if there was an issue with the original run-in, or two people missed the correct dial-in. Thank-you all once again. This BBS is a fantastic resource.
Bayard DeNoie

It's your needles. I heavily modified a motor and went with the stock needles in my HS6s and I had the exact same problem. I live near Joe Curto so I went to him and he set me up w/different needles. Give him a call and tell him what needles you currently have and your problem and he'll set you up. You'll probably need to buy a few different needles as you'll need to find the right fit. I'm still experimenting but put a hold on it as my competition valve springs ruined the cam after a few thousand miles. No, it's not a B motor but a stroked bored cammed etc Datsun Roadster motor. BTW, I run RB needles to great effect.
Mike MaGee

Bayard; I would also think cam since that is what was changed last. I don't know what information is given in a Piper cam card. Most USA cams are timed at a specified opening and closing point at a specified lift, usually .050". I'm sure Peter's timing at maximum lift is very accurate and normally works well. He records the readings at .0005" before and after maximum lift. However, all the dial indicators I have seen do not have a vernier to measure less than .001". If yours does not have that kind of accuracy you could be off a degree or two, but that isn't enough to make it go from hero to zero at 3k rpm. If the cam card gives the open and close points at a specified lift you may want to see what the readings are. Knowing that you can compute the centerline if the cam lobe is symetrical. If the card does not give the opening and closing points you can use say .050" and then compute the centerline to see if it agrees with the timing you previously set. Either method should give accurate cam timing if done correctly. All this assumes that piston TDC was accurately verified before the cam was degreed. I'm sure your problem is frustrating, but on the other hand it is very interesting. Hope it turns out to be simple. Good luck, Clifton
Clifton Gordon

This thread was discussed between 20/09/2003 and 21/09/2003

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