Tach problem?

Motor doesn't want to pull hard. Ran 602 gear race before and switched to 648 gear last night and increased maybe 50 rpms. Another driver said he had exact same problem a couple weeks ago and chased it all over the place, couldn't solve it. Then disconnected his tach and problem was gone. Doesn't run tach now. Motor has been gone thru recently and everything seems to be OK. I've heard of this before but is there a way to check without getting back on racetrack? Next race is our big Fall Nationals and really wasn't wanting to try and fix a problem then of all time. Ideas?

Chassis dyno. Worth the time and money

dist. hookup plugged backwards (if MSD), or timing way off?

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