NickZ
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- Joined
- Oct 31, 2018
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I have been experiencing similar lack of acceleration in 4th gear with my 1972 Combat that was reported by @ILLF8ED in this thread:
www.accessnorton.com
(This is a very long thread but the relevant part begins at post #160.)
The timing side seems to be set properly with all the marks in the right places at TDC, but when I measure the actual valve timing, the results show that the valves are retarded approximately 11* from what is specified.
I measured inlet to be opening at 47* BTDC and closing at 79* ABDC.
For measurement, I set the timing side inlet tappet clearance to .017” and used .0025” feeler gauge to detect when the valve lift begins at .0145” (which is closest to the .013” cam lift specified in the Workshop Manual for this measurement when taking the 1.13:1 rocker geometry into account).
Workshop Manual:
So it appears that I may also have a 1972 Combat motor with a mis-marked timing pinion as did ILLF8ED.
The measured numbers are some what similar to the specs for standard (non-combat) cam (50* BTDC inlet opens; 74* ABDC inlet closes) so I remeasured valve lift using dial gauge on valve collar:
Timing side: inlet .439” (at approx. 70* BBDC); This is within 1% of Combat spec .441"
exhaust .365” (at approx. 82* ABDC); This is 6.6% below Combat spec .391"
Drive side: inlet .444” (at approx. 70* BBDC); This is within 1% of Combat spec
exhaust .368” (at approx. 82* ABDC); This is 5.8% below Combat spec.
These inlet measurement tend to confirm it is a Combat cam, but for some reason the exhaust measurements are considerably lower than Combat cam specs and actually closer to the standard cam spec of .375”, creating doubt.
Can anyone help me explain this?
Do I have a non-standard Combat cam with modified exhaust lobes?
I looked at @dynodave 's cam profiles on his atlanticgreen.com website but i don't have enough measurement precision to conclude anything from those.
I want to be as certain as I can about what I have before I go changing the valve timing set-up by advancing the intermediate gear.
Any help will be appreciated.
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750 Combat top speed problem (2015)
Checking in with the members. My 750 Combat has had an issue for the last couple of years not being able to pull above 80mph in top gear. I've worked through several things from carbs to camshaft - these items all check out fine. Lately I suspected timing although had set this to 28...
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(This is a very long thread but the relevant part begins at post #160.)
The timing side seems to be set properly with all the marks in the right places at TDC, but when I measure the actual valve timing, the results show that the valves are retarded approximately 11* from what is specified.
I measured inlet to be opening at 47* BTDC and closing at 79* ABDC.
For measurement, I set the timing side inlet tappet clearance to .017” and used .0025” feeler gauge to detect when the valve lift begins at .0145” (which is closest to the .013” cam lift specified in the Workshop Manual for this measurement when taking the 1.13:1 rocker geometry into account).
Workshop Manual:
So it appears that I may also have a 1972 Combat motor with a mis-marked timing pinion as did ILLF8ED.
The measured numbers are some what similar to the specs for standard (non-combat) cam (50* BTDC inlet opens; 74* ABDC inlet closes) so I remeasured valve lift using dial gauge on valve collar:
Timing side: inlet .439” (at approx. 70* BBDC); This is within 1% of Combat spec .441"
exhaust .365” (at approx. 82* ABDC); This is 6.6% below Combat spec .391"
Drive side: inlet .444” (at approx. 70* BBDC); This is within 1% of Combat spec
exhaust .368” (at approx. 82* ABDC); This is 5.8% below Combat spec.
These inlet measurement tend to confirm it is a Combat cam, but for some reason the exhaust measurements are considerably lower than Combat cam specs and actually closer to the standard cam spec of .375”, creating doubt.
Can anyone help me explain this?
Do I have a non-standard Combat cam with modified exhaust lobes?
I looked at @dynodave 's cam profiles on his atlanticgreen.com website but i don't have enough measurement precision to conclude anything from those.
I want to be as certain as I can about what I have before I go changing the valve timing set-up by advancing the intermediate gear.
Any help will be appreciated.